Next app architecture steps

Since the last days I read a lot on how to improve the app on a more deeper level.
So here are my rough ideas, which can and should be discussed.

I will list them as an overview and then open new issues to get in more detailed discussion.

- [ ] [switch to modern system](https://github.com/nextcloud/android/issues/3146)
- [x] [switch to WorkManager](https://github.com/nextcloud/android/issues/3147)
- [x] [use Lombok](https://github.com/nextcloud/android/issues/3148)
- [ ] [use dav4android, with okhttp3](https://github.com/nextcloud/android/issues/3149)
- [x] [hard seperation of client / library](https://github.com/nextcloud/android/issues/3150)