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?

• [x] Yes, this is a breaking change.
• [ ] No, this is not a breaking change.

[lesnitsky]

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