feat: anonymous account upgrade with error handling

[russellwheatley]

Notes

1. Set up auto upgrade anonymous account for:

• Signing user in with credential
• Creating user
• Sign in with email link

2. Created merge conflict error so we can pass back the credential to the user should it fail on email-already-in-use like in the current FirebaseUI Auth. This is logic is duplicated in a few place in previous implementation but is handled in one spot for FUIAuth SwiftUI.

Questions

1. Do we want the logic for forcing same device? It isn't a configuration option currently in FUIAuth SwiftUI. See: https://github.com/firebase/FirebaseUI-iOS/blob/main/FirebaseEmailAuthUI/Sources/FUIEmailAuth.m#L829-L844. Personally, I think this might be overkill. At the moment, if email-link isn't in app storage, we throw an error here: https://github.com/firebase/FirebaseUI-iOS/pull/1247/files#diff-659d4c231d40710232200eb4482235de5964068a1395944bc32983fe0ed8670cR305. We could just push user back to email link sign-in View if it isn't present.
2. Do we want to also link provider with email link? In the previous implementation, it does have this ability but only for Facebook as far as I can tell. It checks query param here: https://github.com/firebase/FirebaseUI-iOS/blob/main/FirebaseEmailAuthUI/Sources/FUIEmailAuth.m#L261-L270 but only for Facebook: https://github.com/firebase/FirebaseUI-iOS/blob/main/FirebaseEmailAuthUI/Sources/FUIEmailAuth.m#L296-L359

[morganchen12]

LGTM other than the one comment.

[morganchen12]

We should handle the currentUser null state in a separate conditional, otherwise this do-catch has the possibility of executing nothing and returning no error. From a convenience perspective it may also be nice to pass the unwrapped user through to the AccoutMergeConflictContext so the consumer doesn't have to again unwrap currentUser.

In theory currentUser should never be null here, so throwing an error is appropriate.

[russellwheatley]

@morganchen12 - updated. I didn't add User to the exception as it produced an Xcode compiler error: Stored property 'user' of 'Sendable'-conforming struct 'AccountMergeConflictContext' has non-sendable type 'User'

It seems User is non-sendable.

[morganchen12]

A fix for that is in the pipeline in upstream Firebase, so maybe leave a TODO and just send the user's UID (String) for now?