feat(*): support initializing default FirebaseApp instances from Dart

[russellwheatley]

Description

How it works

• Maintain existing behaviour; Attempt to initialize default app on native, if no google-services.json or GoogleService-Info.plist (or if firebase.initializeApp() is not called from index.html for web ) is available, then dart will not have a default app.
• Additional behaviour; once no default app is available, check if FirebaseOptions have been passed into initializeApp() (without the name parameter). Use these values to initialise default app.
• Maintain existing behaviour; If name & FirebaseOptions are passed in, use these values to create additional app.

Untouched legacy plugins that internally initialise default app:

• firebase_dynamic_links
• firebase_in_app_messaging
• firebase_ml_vision (also deprecated)

Untouched deprecated plugins:

• firebase_admob

Untouched packages as they don't initializeApp() in example:

• firebase_performance
• firebase_ml_custom

Problem packages

1. firebase_crashlytics. iOS requires this run script for your app to function. Your app will fail to build when using the run script (will also fail without) which fails with this error now we've removed GoogleServices.plist:

It appears this upload-symbols script is looking for GoogleServices.plist property GOOGLE_APP_ID (We pass this in dart code now via this property FirebaseOptions.appId). This will probably require an intervention from Firebase Crashlytics team.

Additional config

1. firebase_auth. Using verifyPhoneNumber() API does require an update to the user's ios/Runner/Info.plist, specifically adding the reverse client id as described in the documentation here. Also need to pass in iosClientId in the FirebaseOptions (see this commit).

Related Issues

none.

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]

nit: trailing coma (here and other example apps)

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[Ehesp]

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[Salakar]

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