Test repository REST API folder call

Currently, the REST API contains the "/folders" endpoint which returns the complete hierarchy of the repository.
For very large repositories, with thousands of folders, this will take a long time to return and will also contain a lot of information which may not even be necessary from the user's point of view.

A new endpoint shall be created, to request the "sub-folders" of a given folder.
This will allow for smaller/shorter calls, with less waste since it allows the user to access only a subset of the folders.