[FR]: Support `ACTION_OPEN_DOCUMENT_TREE` Intent

[tomgehrmann]

Some apps require users to select a folder in which to save a file (or multiple files), which is especially important for automated backups (like in Aegis, Stratum, Signal, Threema).

To enable this functionality for OpenCloud as a provider, the ACTION_OPEN_DOCUMENT_TREE intent of the Android Storage Access Framework (SAF) needs to be supported.