openSignInFlow method
Starts a OAuth sign-in flow for provider
using Firebase. The instance of FirebaseAuth will be from the default Firebase App
Unless withApp is used to build an instance
Implementation
Future<User?> openSignInFlow(String provider, List<String> scopes,
[Map<String, String>? customOAuthParameters]) async {
throw UnimplementedError("openSignInFlow() is not implemented");
}