Stable listing of shared items

[achtsnits]

to ensure consistent object listings, meaning that the "listing" of all shared items remains stable, a separate index must be established. Simply storing a prefix for a share and dynamically resolving it is not sufficient for reproducibility.

Note: This is separate from the topic of item versioning (see issue #22), which ensures the content of the item itself remains unchanged.

On the server side, we can adopt the concept of symbolic links (which point to objects in mounted sources).

TBD: we need to determine the best way to share and allow users to download the index, enabling them to resolve it at a later point