[firebase_auth] Expose AdditionalUserInfo to Firebase User

[Nash0x7E2]

Description

This PR exposes the user's additional information. Useful for retrieving whether the user is new to your app, their IDP-specific user data and username (if the provider is Github or Twitter) .

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated CHANGELOG.md to add a description of the change.
• 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 (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[CosmicPangolin]

@Nash0x7E2 @collinjackson

Hey guys - I see some checks failed here that look pretty easy to solve. Really hoping this makes it in, exposing the new user flag is quite helpful for me.

[collinjackson]

Thanks for this PR. The integration test I added is currently failing on iOS because there's no iOS implementation. Would you be open to adding one?

[Nash0x7E2]

Hi,
Unfortunately I do not have a Mac so I am unable to implement this on iOS

[Nash0x7E2]

Is it possible for you to add this implementation? @collinjackson

[collinjackson]

I'm looking at this change more carefully and I think a different approach is needed. The additionalUserInfo object is a return value for an authentication attempt rather than a signed-in user. The authentication methods (signInAnonymously etc) and onAuthStateChanged should return an instance of a new class (perhaps we can call this AuthResult like Android, AuthDataResult like iOS, or UserCredential like React Native Firebase) rather than a FirebaseUser. The new return value class should include properties user and optionally additionalUserInfo to mirror the native SDKs.

Unfortunately changing the return value of the sign-in methods is a breaking change that will affect every developer using this plugin. I'll try to get some feedback on naming internally so we don't have to do this more than once.

FYI @Ehesp

[jeroen-meijer]

I'd be willing to work on the iOS implementation once the naming issue has been resolved and breaking changes are made clear. 👋🏻

[Ehesp]

Hey - We took the UserCredential naming from the Web SDK. This is do-able for Flutter but as Collin mentioned, the credential class needs to be returned from the authentication methods.

[collinjackson]

I got some feedback internally and we're going to call the result class AuthResult.

I went ahead and implemented the changes we've discussed in #1911. @jeroen-meijer I appreciate your generous offer to help; if you could take a quick look at #1911 and let me know if you have any feedback I'd appreciate it.