GRPC Vehicle Proxy Server

The GRPC proxy server speaks to another IVehicleHardware (no matter if it is real or fake) to serve
other GRPC vehicle hardware(s). It can be used as a real GRPC Vehicle
Hardware backend when it is running on the machine that have a direct
access to the real vehicle bus, or it can also be used as a fake backend
for testing.

Test: `atest GRPCVehicleProxyServerUnitTest`
Bug: 266001013
Change-Id: Ifec6e21223986a68ab089a87a9664c1b52601ea2