fix(firebase_auth): Move communication to EventChannels (no-nullsafety)

[ened]

copy of #4643 - opened against the no-nullsafety branch

Description

Similar to #4209, this PR addresses some issues rgd. isolate communication pertaining left-open "channels" to communicate back from native side to Dart.

Instead, the Flutter EventChannel mechanism is now used for the following parts:

• ID Token Changes
• Auth State changes
• Phone Number verification

Related Issues

Closes #3507
Closes #4416

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

Please help testing the PR

Add the following to your pubspec.yaml:

dependency_overrides:
  firebase_auth:
    git:
      url: https://github.com/ened/flutterfire.git
      ref: fix/4416-firebase-auth-no-nullsafety
      path: packages/firebase_auth/firebase_auth
  firebase_auth_platform_interface:
    git:
      url: https://github.com/ened/flutterfire.git
      ref: fix/4416-firebase-auth-no-nullsafety
      path: packages/firebase_auth/firebase_auth_platform_interface

[Salakar]

Closing in favour of #4643 as its unlikely at this stage that we'll be able to ship a non nullsafety change release sorry 🙈 - null safety releases moved quicker than anticipated. Am keen to get #4643 merged though so let us know on there if you need assistance with anything on that PR