feat(firebase_auth): add custom auth domain setter to Firebase Auth

[Andrey-K-Calven]

Description

https://github.com/firebase/flutterfire/pull/11925 allowed, previously web exclusive, authDomain property to be used on mobile platform; however, actually using it has significant limitations:

1. It requires creating a separate named FirebaseApp with authDomain property on top of the [DEFAULT] FirebaseApp generated from GoogleService-Info.plist/google-services.json because there's no way to set authDomain via those files.
2. Changing authDomain requires deleting and recreating the FirebaseApp.

This change addresses those by adding a custom auth domain setter to FirebaseAuth, plugging into the existing corresponding setters in the native implementations of FirebaseAuth. With this setter authDomain can be set and reset at will on the [DEFAULT] FirebaseApp's FirebaseAuth. This allows for custom auth domains to be used with the go-to style of FirebaseApp initialisation on mobile platforms and is consistent with the way this is handled in the native implementations.

Related Issues

expands on https://github.com/firebase/flutterfire/pull/11925

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[russellwheatley]

Hey @Andrey-K-Calven - thanks for the PR! We're discussing internally on how to move this forward, will let you know what results from the discussions 👍

[Andrey-K-Calven]

Hey @Andrey-K-Calven - thanks for the PR! We're discussing internally on how to move this forward, will let you know what results from the discussions 👍

Hi, @russellwheatley. Thanks for looking at this, did you have a chance to have that discussion? We'd like to use this approach for our auth domain needs.

[Lyokone]

Hey @Andrey-K-Calven, we finally had the green light for this API. Do you mind merging with master to be sure that tests are passing with all the latest changes?