signIn abstract method
Starts the interactive sign-in process.
Returned Future resolves to an instance of GoogleSignInAccount for a
successful sign in or null in case sign in process was aborted.
Authentication process is triggered only if there is no currently signed in
user (that is when currentUser == null), otherwise this method returns
a Future which resolves to the same user instance.
Re-authentication can be triggered only after signOut or disconnect.
Implementation
Future<IGoogleSignInAccount?> signIn();