Implements the gRPC MP Client API

[spericas]

Description

Implements the gRPC MP Client API. This API uses interfaces to create client proxies. It introduces two new annotations, @GrpcProxy for injection and @GrpcChannel to associate a channel with a proxy. Client proxies can be injected into CDI beans to access remote gRPC services.

A sample client interface is shown below:

    @Grpc.GrpcService("EchoService")
    @Grpc.GrpcChannel("echo-channel")
    public interface EchoServiceClient {

        @Grpc.Unary("Echo")
        void echo(String request, StreamObserver<String> observer);
    }

A proxy instance can be injected as follows:

    @Inject
    @Grpc.GrpcProxy
    private EchoServiceClient proxyClient;

Documentation

Will be provided in a follow-up PR.

[spericas]

Example has been moved here helidon-io/helidon-examples#65