refactor(ui)!: split flutterfire_ui into multiple packages, add docs and tests

[lesnitsky]

Description

• Refactor auth providers and configurations in order to be able to use OAuth providers from separate packages
• API docs
• Getting started guide
• unit and e2e tests

Related Issues

Closes #8555, #8384, #8091, #7691, #8440, #7908, #8016, #8091, #8104, #9324

Remaining TODOs:

• migration guide
• CI

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 (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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

• Yes, this is a breaking change.
• No, this is not a breaking change.

[lesnitsky]

Closing in favor of #9340, #9341, #9342, #9343