[FR] Make dependency on play-services-auth optional

[samtstern]

See #1856 for a similar request.

As of firebase-auth version 20.0.0 (firebase-bom version 26.0.0) there is no longer any Firebase dependency on Play services. However FirebaseUI still depends on play-services-auth for Google Sign In, which does have this dependency.

We need to refactor the library to make the dependency optional at runtime without crashing.

[wangjiejacques]

Hi @samtstern , do you have a plan about 7.1.0, will it be available in some days, weeks or months?

[samtstern]

@wangjiejacques we don't have any strict timelines here but I'd say "months" is the best estimate.

[samtstern]

@wangjiejacques well that was easier than I thought! As of version 7.1.0 you should be able to use almost all features on devices without Play services. You just need to make sure you're not using Google Sign In or SmartLock (saving credentials / hint selector) as those have hard dependencies.