Fix accessing Nextcloud via Files app under bad networking conditions

Expected behaviour

The cached / downloaded (Make available offline) folders and files should be available from the Files app even when offline for reading and writing. When connectivity is re-established all changes and new files should be synced to the server.

Actual behaviour

If trying to open a folder via the Files app a modal appears stating "You are offline. Check your internet connection and try again", even for folders available in the Nextcloud app (eg. cached folders or folders made available offline). When saving new files in this state those files may go missing after the connection is restored.

Steps to reproduce

1. Mark some folders and files for offline availability and wait for the sync to complete
2. Disconnect any internet connection (WiFi, Cellular, Bluetooth)
3. Try to access previously downloaded files in the app (will work)
4. Try to access the same files via the Files app (won't work)

Reasoning or why should it be changed/implemented?

This behavior makes it difficult to use Nextcloud natively with other apps when on the go, as many apps allow saving & reading directly from/to the Files app and registered providers. Also files saved when offline going missing with no way to move or copy them is really annoying.

iOS version

iOS 14.5 (on iPad Air 3rd Gen)

App version

Nextcloud Coherence for iOS 3.2.0.30

Server configuration

Nextcloud version:

20.0.8 RC 1

Start by reproducing the offline behavior through the Files app after marking folders and files for offline availability, then compare it with access in the Nextcloud app. Trace the Files app provider and offline sync behavior to determine why cached content cannot be opened and why offline-created files disappear; done means offline reading and writing work and changes sync after reconnection.