add grpc interface to kuksa.val

[wenwenchenbosch]

After COVESA/vss-tools#42 is merged, there would be a proto file availiable for vss model. It would be nice to implement a grpc interface for kuksa.val.

[SebastianSchildt]

Questions

• which abstraction (simple or on datapoint level? Is it dynamic)
• security: External components "trusted", or need to use JWT to set up security context
• Interface scope "larger" than Websocket?

How would a good interface look like?
@nayakned @anntexx

[SebastianSchildt]

I am not necessarily convinced it is a good basis for our interface, but for going "deep" into a specific tree, attached is the output of the current vss-tools protobuf converter

vss_rel_2.1-develop.pb.zip

[SebastianSchildt]

Done and redone in #299