new endpoint to work with skeletondirectories used for testing

[individual-it]

Description

endpoint to get the root of testing skeleton directories and to set a skeleton directory below that root

only the SUT knows its true full path of the skeleton directory, it cannot be guessed by the test runner see: https://github.com/owncloud/core/issues/34483

Checklist:

• [x] Latest changes are published according to instructions in README.md