Add configuration for CKAN-based open.rlp

This should work but appears to fail due to access restrictions:

ERROR harvester: Failed to harvest source open-rlp: HTTP status client error (403 Forbidden) for url (https://open.rlp.de/api/3/action/package_search?start=0&rows=100&fq=%2Bgroups%3Aenvi)

They appear to allow only a different API URL, namely https://open.rlp.de/api/action/package_search instead of the versioned https://open.rlp.de/api/3/action/package_search. I adjusted our CKAN harvester and tested all other CKAN-based harvesters and the unversioned URL seems to work everywhere, so I think this is good to go now.