2-way sync of folders which are _outside_ of the local Nextcloud folder

Similar to FolderSync or [EasySync](https://github.com/phpbg/easysync) the idea is to sync an external, not yet folder to Nextcloud server and back.

This is opposite to internal [2way-sync](https://github.com/nextcloud/android/pull/13294).

Use case could be
- sync Notes folder to Nextcloud
- backup any folder to Nextcloud
- …

This issue will be updated to take implementation ideas into account.

Please 👍 if you like this feature, so that we know about the "demand" of it.

Start by reviewing the existing internal 2-way sync pull request (#13294) and comparing the FolderSync and EasySync examples linked in the issue. Clarify how an external folder should be selected, synchronized, and backed up before implementation. Done means the Android app supports the agreed external-folder sync behavior in both directions.

Scritto dal modello di indicizzazione a partire dal testo della issue.