OnDemand files (Cloud Files API / Projected File System) to support partial sync

What

While sync downloads all files locally, mount does not download files normally, despite of the caching.
It would be favorable to have a similar experience like OneDrive / GDrive / OwnCloud Clients where a directory can be synced "virtually". This means that the cloud files are projected into a local directory via placeholders and do not consume storage.
Only files that are needed can be synced to the local storage and consume storage.

Why

This would allow mobile work scenarios where large cloud storages can be partially synced and the user can work on the files.
Currently the only possibilities are syncing the whole storage (much local storage required) or mounting a volume (Not available while not connected to the network).

Start by comparing the existing sync and mount behavior described in the issue, then identify the project entry points that implement each mode. Define how placeholders, on-demand downloads, local caching, and offline access should work before changing code; done means a cloud directory can be projected locally and only requested files consume local storage.