[firebase_auth] Add support for linkWithPopup and linkWithRedirect

[doppio]

The auth package supports linking Firebase accounts via credential and phone number, but not via the pop-up based flow, which is preferred on web. The Firebase User API has a linkWithPopup function which I presume could be exposed through FlutterFire.

My use case is that I have enabled the "One account per email address" setting in my Firebase project. The FlutterFire documentation explains how to handle "account-exists-with-different-credential" errors when using the recommended native flow, but an equivalent approach doesn't seem to be possible exclusively using the popup flow on web.

The same can be said for linkWithRedirect.

[rbozan]

Same goes for linkWithRedirect

[doppio]

Thanks, that's a good point. I've updated the discussion to mention that.

[doppio]

As @darshankawar pointed out to me in #7360, linkWithPopup is now supported (see PR #6562).