[firebase_messaging] Add timeSensitive to iOS NotificationSettings

In iOS, the settings returned by [`getNotificationSettings`](https://github.com/FirebaseExtended/flutterfire/blob/dc4c2c1d249abf214c8ec7d835af18c86d64b2f5/packages/firebase_messaging/firebase_messaging/lib/src/messaging.dart#L132) are missing the property `timeSensitive` added in iOS 15.
I would be grateful for any assistance.