[flutter_remote_config] Expose FetchThrottledException constructor for testability

[ncuillery]

As a FetchThrottledException could occur if the user fetches the configuration too many times, I would like to test my app behavior in such a scenario.

My remote config singleton is mocked with Mockito, so I'm able to mock any of its methods, like:

when(_remoteConfig.fetch(expiration: anyNamed('expiration')))
    .thenThrow(FetchThrottledException(endTimeInMills: Duration(minutes: 5).inMilliseconds));

Unfortunately I can't because the constructor is private: https://github.com/FirebaseExtended/flutterfire/blob/6b6c6404715297c8324b57ece5ecf638431bbd9e/packages/firebase_remote_config/lib/src/remote_config_fetch_throttled_exception.dart#L9

Is it possible to open that up? I don't think it would hurt POO encapsulation too much.