Add a fetch endpoint for directories

[imsdu]

Motivation
With #4924, directories can now be created via a link operation and we need an endpoint to fetch it in the same manner as the other entity

Acceptance criteria

1. Access permission is performed
2. It is possible to fetch the latest revision, at a given tag or at a given revision
3. Like other entities, proper errors are returned when the revision or the tag does not exist

This section should be about results in domain terms, not implementation details.

Implementation notes
Add any technical details which might be helpful here. Which classes will need to change? What things do you need to know about which might not be obvious?

API changes
Example request/response if applicable.

Mockups
Screenshots or XD export if applicable.

Not in scope

1. The X will not be...
2. The Y will be handled separately...