When the user is known upon signIn, remove friction.

* Do not ask for user selection again in the authorization popup
* Pass the email of the known user as a hint to the signIn method

Author: ditman

packages/google_sign_in/google_sign_in_web/lib/src/gis_client.dart

@@ -196,9 +196,15 @@ class GisSdkClient {
   ///     [_initialScopes], so we can retrieve User Profile information back
   ///     from the People API (without idToken). See [people.requestUserData].
   Future<GoogleSignInUserData?> signIn() async {
+    // If we already know the user, use their `email` as a `hint`, so they don't
+    // have to pick their user again in the Authorization popup.
+    final GoogleSignInUserData? knownUser =
+      utils.gisResponsesToUserData(_lastCredentialResponse);
     // This toggles a popup, so `signIn` *must* be called with
     // user activation.
     _tokenClient.requestAccessToken(OverridableTokenClientConfig(
+      prompt: knownUser == null ? 'select_account' : '',
+      hint: knownUser?.email,
       scope: <String>[
         ..._initialScopes,
         // If the user hasn't gone through the auth process,