Allow passing full GoogleSignInOptions to auth

[samtstern]

Requested in #757 and #1021

Would allow more flexibility than just scopes.

[samtstern]

Also related to #806

[guillermo-varela]

Hi,

Are you guys working on this or do you need a PR?

Thanks.

[samtstern]

@guillermo-varela we have not yet started working on this. If you plan to submit a PR (which would be really cool!) could you show the proposed API here first? As this would change the public API it needs a little more discussion than your average PR.

[samtstern]

This has been fixed and released in version 3.2.0.