Expose NSError.userInfo in FirebaseAuthException for Apple target #781
Description
Right now in my app a FirebaseAuthUserCollisionException is being thrown when trying to link an anonymous account that already signed in with a provider.
On Android the FirebaseAuthUserCollisionException has a updatedCredential property, which is an AuthCredential we can use to link/sign in with.
However on iOS, only the error message is exposed. If we had access to the NSError.userInfo we could also get the updated credential via NSError.userInfo[AuthErrorUserInfoUpdatedCredentialKey]
I attempted to fork and add support myself, but I couldn't get the project to sync on my machine.