🐛 [firebase_app_check] `.getToken()` should return a `AppCheckToken` class

[nilsreichardt]

Currently, the await FirebaseAppCheck.instance.getToken() method has a String? return type. But should return a AppCheckToken class like the native SDKs does. The reason for this is that this class has also attributes like expireTimeMillis: https://firebase.google.com/docs/reference/js/app-check.appchecktoken#appchecktokenexpiretimemillis.

This would be require a breaking change.