GRPC connectors

[luca-domenichini]

Would be good to have grpc connectors, both client and server style.

Server connector should bring up a grpc service that allows clients to request data on demand.
It should also provide a streaming method to have events pushed to clients asynchronously.
This package (or another one?) should also provide a client library to be easily used by external processes

Client connector should do the same but the opposite way.
This package (or another one?) should also provide a grpc service to be installed in an external application that wants to act as the grpc service server. External clients, at this point, should query that grpc service for data/status/streaming etc...

Maybe client and server packages can be the same in both connector kind, be that SmartIOT -> grpc client -> grpc server or SmartIOT -> grpc server -> grpc client... maybe some endpoints would be useless.. must investigate that.