[ui] split flutterfire_ui into multiple firebase_ui_* packages

FlutterFire UI should be split into several packages:

- firebase_ui_auth
- firebase_ui_database
- firebase_ui_firestore
- firebase_ui_localizations
- firebase_ui_oauth
- firebase_ui_oauth_apple
- firebase_ui_oauth_facebook
- firebase_ui_oauth_google
- firebase_ui_oauth_twitter