Provide gRPC examples

[RafalSkolasinski]

It could be beneficial for first-time user if the main examples, e.g. mnist-sklearn, contained in addition to REST a gRPC request code for example using grpcurl.

Another idea, probably to track separately, would be to provide a CLI functionality to send gRPC request without need to user grpcurl (pointing it to protos usually is not very convenient).

[dtpryce]

+1