feat(firebase_ui_auth): Override the default deletion modal

[EArminjon]

Description

Add a callback to let developers override the default account deletion modal.

Related Issues

https://github.com/firebase/FirebaseUI-Flutter/issues/492

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.

• [X] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [X] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• [X] All existing and new tests are passing.
• [X] I updated/added relevant documentation (doc comments with ///).
• [X] The analyzer (melos run analyze) does not report any problems on my PR.
• [X] All unit tests pass (melos run test:unit:all doesn't fail).
• [X] I read and followed the Flutter Style Guide.
• [X] I signed the CLA.
• [X] 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.
• [X] No, this is not a breaking change.

[github-actions[bot]]

Hello 👋, this PR has been opened for more than 2 months with no activity on it.

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[EArminjon]

Get out bot.

[github-actions[bot]]

Hello 👋, this PR has been opened for more than 2 months with no activity on it.

If you think this is a mistake please comment and ping a maintainer to get this merged ASAP! Thanks for contributing!

You have 15 days until this gets closed automatically

[EArminjon]

Get away bot