Google Sign-in Support in Auth Sample

Hi guys,

I love Firebase, but it's pretty important to be able to use google sign-in easily. Unfortunately both the documentation and these samples give no guidance there. It's a bit of a documentation hole.

The auth example given in this repo is email+password, which isn't as useful as a google sign-in, which is quicker and users are more likely to use. Sadly the [documentation](https://firebase.google.com/docs/auth/unity/google-signin) just forwards you to the native Android page to solve this. As the example given in the android page uses a new activity it's non trivial to implement either in Unity or as a separate module.


It would be incredibly useful if a practical example was made and/or a separate aar module that can be used from within Unity for launching the sign-in with a call back that returns the required token.

All the best. And thanks for making such a great library.