[firebase_messaging] add support for critical alerts on iOS

[KaiBroeker]

Hello,
in some cases it is important to generate notification that also sounds if the phone is on dnd or silent.
I searched a while and found the old issue #2033 and a good tutorial

The most part of the tutorial have to be done manually.
But hopefully we can add the permission requesting on the device in firebase.
As workaground I added the code from the tutorial manual in the AppDelegate:

if #available(iOS 12.0, *) {
    authOptions = [.alert, .badge, .sound, .criticalAlert]
} else {
    authOptions = [.alert, .badge, .sound]
}
UNUserNotificationCenter.current().requestAuthorization(options:   
  authOptions!) { (granted, error) in
    if !granted {
        print(“The application requires Notifications permission to   
         display push notifications. Please enable it in settings.”)
    }
}

That works ok but I think there is a better way.
If this would be intergrated in firebase, we can handle the "not granted" case in flutter and don't have to make durty implementations and work arounds.
Maybe after adding this, we also can add a small documentation for others who need this feature.

Thanks!
Kai

[KaiBroeker]

in the code I maybe found that its already implemented.
But not well documented:

  if (Platform.isIOS) {
    FirebaseMessaging messaging = FirebaseMessaging.instance;
    NotificationSettings settings = await messaging.requestPermission(
      alert: true,
      announcement: false,
      badge: true,
      carPlay: false,
      criticalAlert: true,
      provisional: false,
      sound: true,
    );
    print('User granted permission: ${settings.authorizationStatus}');
  }