protoc-gen-go-dubbo 改进建议

[XiaoWeiKIN]

grpc：

// UnimplementedSpecialServer can be embedded to have forward compatible implementations.
type UnimplementedSpecialServer struct {
}

func (*UnimplementedSpecialServer) GetName(context.Context, *SpecialRequest) (*SpecialReply, error) {
	return nil, status.Errorf(codes.Unimplemented, "method GetName not implemented")
}
func (*UnimplementedSpecialServer) SayHello(context.Context, *SpecialRequest) (*SpecialReply, error) {
	return nil, status.Errorf(codes.Unimplemented, "method SayHello not implemented")
}

dubbbo-go：

type SpecialProviderBase struct {
	proxyImpl protocol.Invoker
}

xxxProviderBase 也默认的实现下接口的方法，不然业务代码需要把接口放方法全部实现

[LaurenceLiZhixin]

Refactor of protoc-gen-go-dubbo3grpc needs help, which is needed to be fixed at :https://github.com/dubbogo/tools/tree/master/cmd/protoc-gen-dubbo3grpc

[LaurenceLiZhixin]

refer to: #1460 (comment)