Please add google signin and play games signin to this repo

[Thaina]

Or please at least update these old repo to be newer version and make it compatible with UPM

https://github.com/googlesamples/google-signin-unity
https://github.com/playgameservices/play-games-plugin-for-unity

But I think these crucial package should be maintain here too

I have also made a working pull request for these packages, which should be easily moved into this repo

https://github.com/Thaina/google-signin-unity/tree/newmigration
https://github.com/Thaina/play-games-plugin-for-unity