Mockgen Generate Code for GRPC Interface

gistfile1.txt

Due to the method of `UnimplementedUserServiceServer` interface isn't exported, we need to run 
the shell here to manipulate the generated file again:

```shell
sed -i '' '/type MockUserServiceServer struct {/,/}/ { 
    /type MockUserServiceServer struct {/!b
    a\
    \tholepb.UnimplementedUserServiceServer
}' proto/mockedpb/api_grpc.pb.go
```

This command will append the default unimplementation into the generated structure:
```diff
// MockUserServiceServer is a mock of UserServiceServer interface.
type MockUserServiceServer struct {
+	holepb.UnimplementedUserServiceServer
	ctrl     *gomock.Controller
	recorder *MockUserServiceServerMockRecorder
}
```