Add subscribe to VHAL proto.

The latest IVehicleHardware adds Subscribe/Unsubscribe function
to replace the existing updateSampleRate. This CL adds the
Subscribe function to protobuf definition and implements it.

Test: atest GRPCVehicleHardwareUnitTest GRPCVehicleProxyServerUnitTest
Flag: EXEMPT hal change
Bug: 328316981
Change-Id: I4f02885b77f21a215a8b282be583f76118e400ba