More convenient way to register grpc service

[ArtemyGordon]

This PR simplifies how to register gRPC services

Previously, we had to create the service implementation and then wrap it into the generated *Server type manually:

let grpc_service = GrpcServiceImpl::new(app_context.clone());

service_context.configure_grpc_server(|builder| {
    builder.add_grpc_service(GrpcServiceServer::new(grpc_service.clone()));
});

Now we can register the service implementation directly:

service_context.configure_grpc_server(|builder| {
    builder.add_service(GrpcServiceImpl::new(app_context.clone()));
});

This works when GrpcServiceImpl is generated via the generate_grpc_service macro, which also implements the IntoGrpcServer trait for GrpcServiceImpl (so the builder can automatically wrap it into the corresponding *Server type under the hood)