add a service regsitry when generate source code

[gzzyj]

A lazy and common way for gRPC service registration. I can use the registry to invoke the grpc.Server's function (func (s *Server) RegisterService(sd *ServiceDesc, ss interface{}) ) at the special time. It is more convenient to manage the registration than directly to invoke the pkgName.RegisterXXXServer( s *grpc.Server ,svr XXXServer)

[gzzyj]

1. add a serviceoption
2. handle the serviceoption in code gen plugin named grpc。export the serviceDescription