(Experimental feature) Expose the location/path of the physical file via the Access api

Same idea as behind the existing direct upload, but simplified and generalized for all datafiles.
/api/access/datafile/{id} downloads the bytes
/api/access/datafile/{id}/storageLocation outputs something like /mnt/files/data/xxxxxxxx (for a filesystem-stored datafile).
The use case is Dataverse deployed on a cluster along with other nodes all sharing block access to the data objects; a computational node serving as an "external tool" will be able to run on a big data file directly without having to stream it via the Dataverse API.
May be needed for the MOC-POC demo.

Implementation should be trivial/straightforward.

Start by tracing the existing direct-upload Access API implementation and the handlers for /api/access/datafile/{id}. Check how filesystem-stored datafiles expose their storage location and how the response should behave for other storage types. Done means the storageLocation endpoint returns the expected physical path for supported datafiles without changing the existing download endpoint.