Vehicles: Implement "mount/unmount vehicle"

[MeFisto94]

• Add a new packet that is sent when entering and leaving a vehicle
• This needs to build upon Client: Entity Back Reference on NetworkedEntityId #2, so it can actually send an enter packet instead of force-spawning
• The current behavior of only spawning the entity should also send the new entering packet right after
• When the client receives a relayed "mount vehicle" packet for other entities, the NPCs should also take a seat.
• Backend may need to understand that to stop teleporting the NPC in that case

[MeFisto94]

Unmounting a vehicle is already implemented, but without an entity id, since it still lacks #2