“IMPROVED SUSPENSION SYSTEM FOR A HEAVY VEHICLE” CROSS-REFERENCE TO RELATED APPLICATIONS This patent application claims priority from Italian patent application no. 102020000025021 filed on 22/10/2020, the entire disclosure of which is incorporated herein by reference. TECHNICAL FIELD The present invention concerns a suspension system for a heavy vehicle, in particular a suspension system for a heavy vehicle provided with an electronic traction. BACKGROUND OF THE INVENTION Heavy vehicles such road transport vehicles, e.g. trucks, are more and more electrified for reducing their polluting emissions. In particular, such vehicles may be hybrid vehicles or purely electric vehicles. In particular, the electrified vehicles comprises at least an electric drive rear axle, i.e. a rear drive axle provided with electric machines such as motors/generators configured to impart a torque on the axle to allow its motion or to recharge battery modules of the vehicle when this latter is braking. Such battery modules are usually placed in the proximity of the electric machines of the electric axle in order to provide energy exchange with electric machines in an efficient way. However, as known, an axle of a vehicle is connected to the chassis of this latter via a suspension system configured to dampen the forces acting between the road and the chassis. If a vehicle is provided with an electric axle as described above, the presence of the electric batteries increases the encumbrance around the chassis. Accordingly, known suspension systems for heavy vehicles are not suitable because of the presence of the electric axle. Therefore, the need is felt to provide new arrangement of suspension system that can be mounted to an electric axle and that provides at least the same mechanical properties of the known suspension systems. An aim of the present invention is to satisfy the above mentioned needs in a compact, optimized and cost effective way. SUMMARY OF THE INVENTION The aforementioned aim is reached by a suspension system and a vehicle as claimed in the appended independent claims. Preferred embodiments of the invention are realized according to the claims dependent on or related to the above independent claim. BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the present invention, a preferred embodiment is described in the following, by way of a non-limiting example, with reference to the attached drawings wherein: ● Figure 1 is a perspective view showing a suspension system according to the present invention; ● Figure 2 is a bottom view of the suspension system of figure 1; ● Figure 3 is a lateral view of the suspension system of figure 1; ● Figures 4 and 5 are respectively a bottom and top view of a portion of the suspension system according to the present invention; ● Figure 6 is a lateral view of the portion of suspension system of figures 4 and 5; ● Figures 7 and 8 are respective enlarged perspective views of connections between elements of the suspension system according to the invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1 discloses a suspension system 1 for connecting an axle 2 of a heavy vehicle to a chassis 3 thereof. The chassis 3 comprises, as per se known, a pair of side members 4, 5 extending along a longitudinal axis A of the vehicle and transversally spaced apart. It is noticed that the suspension system 1 may be provided for a work vehicle comprising a front axle (not shown) and either a single rear axle 2 (see figure 4,5 and 6) or two rear axles 2 (Figures 1, 2, 3). In case of presence of two rear axles 2, the second rear axle 2 is placed in a front position along axis A with respect to the first rear axle 2. Clearly, each between the first and second rear axles 2 may be provided as a tag axle 2'' or an electric drive axle 2' and in particular, at least one of the two rear axles 2 may be provided as an electric drive axle 2'.. The electric drive axle 2' is provided with electric machines that are fed by the at least one battery module (not shown) to provide torque to the wheels carried by the electric drive axle 2' or to receive electric energy generated by the electric machines when the vehicle is braking. Due to the presence of such electric machines, the electric drive axle 2' has a greater extension in the space laterally comprised between side members 4, 5 along axis A, both forward and rearward with respect to a standard axle 2''. Accordingly, a first and a second suspension systems 1 are provided between the chassis 3 and, respectively, first rear axle 2 and second rear axle 2. Making reference to only the first suspension system 1, since the second is equal to this latter, it comprises a left side 1'' and a right side1'. Since the left side 1'' and the right side 1' are symmetrically equal with respect to longitudinal axis A, only the right side 1' will be described for sake of brevity. With reference to figures 2 and 6, suspension system 1 comprises a metallic beam 7, i.e. a spring beam, comprising a front terminal portion 7a and a rear terminal portion 7b extending along longitudinal axis A. The metallic beam 7 is connected to axle 2, as described below, in an intermediate portion 7c longitudinally comprised between front and rear terminal portions 7a, 7b. Each the front and rear terminal portions 7a, 7b are connected to chassis 3, i.e. to side member 5, via an air spring such as an air bellow 8 interposed in a vertical direction between the chassis 3 and the metallic beam 7. In particular, air bellow 8 comprises an upper portion 8a configured to be connected to chassis 3, e.g. via a bracket 9, and a lower portion 8b that is connected, e.g. via a threaded element, to the respective front or rear terminal portion 7a, 7b. As per se known, each air bellow 8 comprises an air bag 8c carried between the upper and lower portions 8a, 8b and configured to house pressurized air to dampen relative movements between such upper and lower portions 8a, 8b. The metallic beam 7 is furthermore connected to chassis 3, i.e. to side member 5, via a damper 11 interposed along the aforementioned vertical direction. In particular, the damper 11 is substantially vertical with respect to side member 5 / to ground. Preferably, the damper 11 is a hydraulic damper, i.e. a cylinder, comprising a housing 11a that is carried by side member 5 and a stem 11b that is connected to the metallic beam 7. As per se known, the stem 11b is configured to move inside housing 11a against the action of a fluid contained in the housing 11a to dampen relative movement between the side member 5 and metallic beam 7. The stem 11b is connected to metallic beam 7 in a portion comprised between the intermediate portion 7c and one between the front terminal portion 7a or the rear terminal portion 7b, as specified in the following description. In particular, the stem 11b is connected to the metallic beam 7 via a hinge connection 12 configured to allow a rotation of terminal portion of stem 11b about an axis parallel to a transversal axis B that is the longitudinal axis of axle 2. The casing 11a is instead connected to side member 5 via a hinge connection 13 configured to allow a rotation of terminal portion of casing 11a about an axis parallel to longitudinal axis A. The metallic beam 7 is connected to the respective axle 2 via a connection assembly 15 preferably comprising at least a plate 16 vertically interposed between the axle 2 and the intermediate portion 7c of metallic beam 7. The plate 16 defines a top portion configured to cooperate in contact with an external surface of axle 2 and a lower portion configured to cooperate in contact with an outer surface of metallic beam 7. The connection assembly 15 further comprises retaining means 17 configured to exert a compression force between the axle 2, the plate 16 and the metallic beam 7 so as to maintain this latter connected together by friction. In particular, the retaining means 17 may comprises two U-shaped brackets 18 provided with threaded terminal ends configured to pass through the intermediate portion 7c of metallic beam 7 and threaded elements configured to cooperate with such threaded terminal ends to exert the compression force between the aforementioned elements. Suspension system 1 further comprises a transversal retaining element 20 configured to support transversal movements acting between the axle 2 and chassis 3 but allowing vertical movement between these latter elements. In particular, the transversal retaining element 20 comprises a V-shaped rod 21 provided with a first arm portion 21' and a second arm portion 21'' each provided with a first portion and a second portion. The first portions are stably connected together while the second portions are spaced along transversal direction getting over from the first portion connection. Accordingly, the first and second arm portions 21', 21'' are inclined in a horizontal plane with respect to longitudinal axis so that each between their second portions gets closer to a respective side member 4,5. In greater detail, the first and second arm portions 21', 21'' are symmetrical with respect to longitudinal axis A. Advantageously, the first arm portion 21' and the second arm portion 21'' are realized as one piece. The first portions thereof are connected to the axle 2 via a hinge connection 22 that allows the rotation of such first portions about an axis parallel to transversal axis B. More preferably, the first portions are connected to axle 2 in an upper portion thereof, more preferably via a flange 23 carried by the axle 2. The second portions of the first arm portion 21' and the second arm portion 21'' are connected by a respective hinge connection 24 to the chassis 3. In the disclosed embodiment, the hinge connection 24 is realized with a flange element 25 carried in a cantilevered way from a respective side member 4, 5. Such flange member 25 is fixed to the respective side member 4, 5 via fixing element such as threaded elements. The suspension system 1 further comprises stop means 27 configured to avoid an excessive vertical motion of the axle 2 with respect to chassis 3. In particular, such stop means 27 comprise a flange 28 fixedly carried by chassis 3 , e.g. by an intermediate element (not shown) carried by side members 4, 5 and configured to cooperate at contact with outer surface of axle 2 when its motion exceed a predetermined height. Advantageously, the suspension system 1 comprises a pair of stop means 27 housed in the space laterally comprised by side members 4, 5 and symmetrically disposed with respect to longitudinal axis A. Preferably, the flange 28 may be provided with rubber buffers to avoid a direct metal-to- metal contact between the axle 2 and flange 28. Suspension system 1 further comprises a longitudinal retaining element 30 configured to support longitudinal movements acting between the axle 2 and chassis 3 but allowing vertical movement between these latter elements. In particular, longitudinal retaining element 30 comprises a rod 31 connecting axle 2 to chassis 3 via metallic beam 7. In particular, the rod 31 connects axle 2 to a front hanger 32 fixedly carried by side member 4,5. In greater detail rod 31 comprises a first terminal portion 31a and a second terminal portion 31b along axis A and an intermediate portion 31c comprised between these latter. The first terminal portion 31a is connected to the front hanger 32 via a hinge 33 and the second terminal portion 31b is connected to metallic beam 7 via a further hinge 33. Both hinges 33 are configured to allow a motion about an axis parallel to transversal axis B of the respective terminal portion 31a, 31b. It is noticed (see figure 2) that the rod 31 is completely placed laterally outside with respect to the space comprised by side member 4,5. Front hanger 32 comprises an upper portion 32' configured to allow the fixation of the front hanger 32 to the respective side member 4,5 and a lower portion 32'' configured to allow the connection of the rod 31 to the same. The upper portion 32' defines a plurality of holes 34 configured to allow the fixation of the front hanger 32 to the side member 4,5 and has a longitudinal extension greater with respect to lower portion 32''. In other words, the intermediate portion 32''' connecting upper and lower portions 32', 32'' has a tapered cross-section along longitudinal axis A. Preferably, the hinge 33 between rod 31 and lower portion 32'' of front hanger 32 is realized between a pair of terminal legs extending spaced apart from lower portion 32'' defining a space for accommodating the rod 31 and providing a support for hinge 33. The suspension system 1 further comprises a stabilizer bar 35 configured to dampen possible torque acting between the axle 2 and chassis 3. The stabilizer bar 35 comprises essentially (as best shown in figure 4) a substantially straight portion 35' and a pair of terminal portions 35''. The terminal portions 35'' are connected to the straight portion 35' via respective curved portions 35'''. The stabilizer bar 35 is placed underneath the axle 2 and the metallic beam 7. Advantageously, the straight portion 35' is carried by metallic beam 7 and the terminal portions are carried by the front hanger 32. Furthermore, the terminal portions 35'' extends substantially perpendicular with respect to the straight portion 35', i.e. long longitudinal axis A and are both placed on the same side with respect to this latter. Therefore the stabilizer bar 35 defines a substantially U-shaped path in a horizontal plane wherein the straight portion 35' is placed between the air bellows 8 and the respective axle 2. The curved portions 35''' are positioned (see figure 2) externally with respect to side members 4,5 so that they can swivel without interfering with the electric axle 2'' or any further element attached to side members 4,5 such as battery modules. In greater detail, the straight portion 35' is carried by metallic beam 7 via bearing means 36. In particular, bearing means 36 comprises a first half 36' that is realized fixedly carried, preferably realized as one piece, by intermediate portion of metallic beam 7 and a second half 36'' that can be selectively fixed to the first half 36' to allow mounting and detaching of the stabilizer bar 35. More preferably, both the first and second halves 36', 36'' are realized in metallic material. The baring means 36 may comprised two half shells (not shown in detail) realized in polymeric material, e.g. rubber, that are fixed to the respective half 36', 36'' through the fixation of these latter together. In particular, the bearing means 36 are two, located at extremities of straight portion 35', i.e. in proximity of the curved portions 35'' and configured to allow a rotation of stabilizer bar 35 about an axis parallel to transversal axis B. The terminal portions 35'' are each connected to the chassis 3 via a connection assembly 38 configured to allow a movable connection, in particular a swiveling motion about an axis parallel to transversal axis B. In greater detail the connection assembly 38 comprise a connection element 40 provided with a lower end connected to terminal portion via a hinge 39 and an upper end connected to chassis 3 via a further hinge 39. Hinges 39 are both configured to allow a rotation about an axis parallel to transversal axis B. Preferably the hinge 39 between the upper end of connection element 40 and chassis 3 is realized between such upper end and the front hanger 32. In greater detail the front hanger 32 defines a support portion 41, extending cantilevered in the space comprised between the side member 4,5 . In particular, the support portion 41 is a flange that extends from intermediate portion 32''' of front hanger 32 in proximity of upper portion 32'. Preferably support portion 41 is realized as one piece with front hanger 32 to which the connection element may be mounted into a detachable way. Preferably, the connection element 40 comprises a pair of arms 40', 40'' placed laterally with respect to the terminal portion 35'', therefore spaced apart along transversal direction, and hinged at respective terminal ends to hinges 39. In the described double rear axles configuration, the front hanger 32 is common and allows the connection of the longitudinal rod 31 and of stabilizer bar 35 of both suspension systems 1. Furthermore, in the two axles 2 configuration further two possibilities may be envisaged, wherein the first rear axle 2 is a drive axle and the second rear axle 2 is a tag axle, as shown in figure 1 and wherein both the first and second rear axles 2 are drive axles, as shown in figures 2 and 3. The difference between this two embodiments lies in the position of the damper 11 and of the opening of the U-shaped stabilizer bar 35. Indeed, in the configuration of figures 1, the dampers 11 are both placed in a rear position with respect to the respective axle 2 and the U-shaped stabilizer bars 35 are both opened towards a front portion of the vehicle, i.e. they have the same disposition. In such configuration the central common front hanger 32 supports a single stabilizer bar 35 while the other is connected directly to a portion carried fixedly by the side elements 3, 4. Instead, in the configuration of figure 2, the dampers 11 are placed externally with respect to the respective axles 2, i.e. in a rear position with respect to first rear axle and in a front position with respect to the second rear axle along axis A. In such configuration the stabilizer bars 35 are positioned so that they are opened in an opposite direction, i.e. they are symmetrically opposed with respect to a transversal axis B. In other words, the openings of their U-shaped cross-section are faced one with respect to the other. In such configuration the central common front hanger 32 supports both stabilizer bars 35. The operation of the above described suspension system 1 is the following. During the motion of the vehicle the at least one axle 2 follows the asperity of the road and may move with respect to the chassis 3 of the vehicle. In particular, at least part of the motion torque is imparted to the vehicle via the electric axle 2'. Such motions are dampened by suspension system 1, in particular: ● Vertical motions are dampened by dampers 11 and air springs 8. Excessive vertical motions are stopped by stop means 27; ● Transversal and longitudinal motions are compensated by V-rod 21 and longitudinal rod 31; and ● Torque motions are compensated by stabilizer bar 35. In view of the foregoing, the advantages of the above described suspension system according to the invention are apparent. The proposed suspension system 1, in particular the position of the stabilizer bar 35, allows the mounting of the suspension 1 also in case of electric axle 2'' while maintaining good mechanical properties. Furthermore, the proposed suspension system 1 is particularly compact. Moreover, the presence of the connecting assembly 38 allows the connection of the stabilizer bar 35 to a substantially standard front hanger 32. The peculiar disposition of the stabilizer bar 35 between the front hanger 32 and the metallic beam 7 allows to use a standard single-piece stabilizer bar 35. Similarly, the rod 31 and the V-rod 21 are standard elements and the bearings/hinges are of standard typology. Accordingly, the suspension system 1 is particularly cheap. Moreover, bearing 36 allows an easy mounting of the stabilizer bar 35 and provides a good damping during its swiveling. As demonstrated, the proposed suspension system 1 may be used for different vehicle configuration, i.e. single, double, drive-drive or drive-tag or tag-drive configuration. Accordingly, the proposed suspension system allows a scale economy, since it can be used for different vehicle configurations. It is clear that modifications can be made to the described suspension system which do not extend beyond the scope of protection defined by the claims. For example, presence position and shape of the elements of the suspension 1 may vary within the limit of the below claims. Moreover, the suspension system 1 may be provided for front or rear axles configured as a drive or a tag axle or as a standard and an electric axle. The shown and described fixation means, hinges and connections may be vary and be substituted by equivalent means.