onStart method
Callback when this executor is started. Returns true if successfully started, false otherwise.
Implementation
@override
Future<bool> onStart() async {
  // create a recurrent timer that checks the conditions periodically
  timer = Timer.periodic(configuration!.period, (_) {
    if (configuration!.triggerCondition != null &&
        configuration!.triggerCondition!()) {
      onTrigger();
    }
  });
  return true;
}