Support Apple auth

[gnprice]

This will be a requirement for launch, because Apple makes it a requirement for the App Store. (That doesn't affect the beta, which is in TestFlight and not in the App Store.)

I doubt any significant number of people are actually using this feature, so there's probably no need for it in the beta period. But we'll see if we hear feedback asking for it.

As @chrisbobbe found at #36 (comment) , there's a "Flutter Favorite" plugin for this: sign_in_with_apple.