Implement the get_rdf_private_datasets_with_access_token feature

First really raw but working test version that implements the possibility to load private rdf datasets with access tokens from the GitLab RWTH instance. The code isn't commented out sufficiently and the procedure logic needs a clean rework since there are a lot of edge cases.

requested review from @manuel.rexer

assigned to @sebastian.neumeier

mentioned in commit 310fcb2b

merged