Protobuf is deprecated

[Smidra]

Hello,
protobuf from 'github.com/golang/protobuf' is deprecated and should be superseded by 'google.golang.org/protobuf'. It possible the team missed to replace the new dependency in some files. Like this, all projects will have the deprecated package as dependency.

Please check If my changes don't break code. I am not sure if the empty packages are compatible.

In gprc_stdio.go

empty "google.golang.org/protobuf/types/known/emptypb"

And in plugin_test.go

empty "google.golang.org/protobuf/types/known/emptypb"

Thanks a lot for your work. Appreciate it.

[alliasgher]

Covered in #377 (combined with the Ping fix).