FR: Add Sign In with Apple Account Linking Sample Code/Documentation

### Feature proposal

* Firebase Component: Auth

While the Firebase documentation is awesome, it is unfortunately very sparse for: [implementing account linking with Sign In with Apple](https://firebase.google.com/docs/auth/ios/apple#reauthentication_and_account_linking). The documentation states to handle `FIRAuthErrorUserInfoUpdatedCredentialKey` though it's hard to understand how exactly.

There is great Firebase documentation for other providers such as [Twitter](https://firebase.google.com/docs/auth/ios/twitter-login#handling-account-exists-with-different-credential-errors), but not for Apple.

I tried looking for how to do this by looking at open-source [FirebaseUI-iOS repo](https://github.com/firebase/FirebaseUI-iOS). However, I don't see how the error `FIRAuthErrorUserInfoUpdatedCredentialKey` is handled there.

Could you please add sample code to show how to do this? Or maybe, provide a link here to the place that [FirebaseUI-iOS repo](https://github.com/firebase/FirebaseUI-iOS) handles account linking with Apple?