Abstract class for auth0 login - for medium article

auth_manager.dart

abstract class AuthManager {
  static AuthManager _instance;

  static AuthManager get instance {
    if (_instance == null) {
      if (kIsWeb) {
        _instance = Auth0ManagerForWeb();
      } else {
        _instance = Auth0Manager();
      }
    }
    return _instance;
  }
  
  Future<User> login([Map<String, String> authResponse]);
  Future<User> userFromTokens(String idToken, String accessToken);
  Future<void> logout();
}