[FEATURE] Navigate between cards programmatically

[lucaloiacono]

Is your feature request related to a problem? Please describe.
It would be great to have the ability to navigate between cards programmatically, using my own logic instead of the standard flow, here there is one use-case:

• User is forced to change password or password has expired: currently i can only deny the user to login and show a message like:Your password has expired. Please click on "Forgot password?" and follow the procedure to set a new one.
  It works but, you know, that's not so good in terms of user experience.
  I'd like to redirect the user to the reset-password card directly when then backend claims the user needs to change password.

Describe the solution you'd like

• Make FlutterLoginState public
• Make _changeCard method into lib/src/widgets/cards/auth_card_builder.dart public
• Add new public method in FlutterLoginState

void changeCard(int index) {
    return authCardKey.currentState!.changeCard(index);
}

• Now we can declare the GlobalKey in our class

class LoginScreen extends StatelessWidget {
  final GlobalKey<FlutterLoginState> flutterLoginKey = GlobalKey();
...

And call the method wherever we want, e.g:

Future<String?> onLogin(LoginData data) async {
  // Auth logic
  ...
  if (passwordExpired) {
    flutterLoginKey.currentState!.changeCard({card_index});
  }
}

Additional context