Use case

The GoogleSignIn APIs for Android, iOS, and Web all provide a mechanism to request additional scopes after sign-in. It would be helpful for the google_sign_in plugin to also provide a mechanism to check granted scopes and grant new scopes (i.e. optional scopes or scopes for new features).

Proposal

Offer two new APIs on GoogleSignIn:
Future<bool> hasGrantedScope(String scope); Which indicates if scope has already been granted by currentUser.
Future<bool> requestScope(String scope); Which requests scope be granted by the currentUser.