Enable GET for container endpoints

[dennisvang]

This enables GET requests to container endpoints, such as catalog/, dataset/, catalog/<id>/dataset/, and so on.
Paging is optional, depending on the presence of query parameters page and size.
The original page endpoints are now deprecated, but we keep them around for backward compatibility.

• added GET controller for the following paths:
  • "{childPrefix}/"
  • "{oUrlPrefix:[^.]+}/{oRecordId:[^.]+}/{childPrefix}/"
• deprecated the GET controller for the page paths:
  • "page/{childPrefix}"
  • "{oUrlPrefix:[^.]+}/{oRecordId:[^.]+}/page/{childPrefix}"
• refactored controller logic:
  replaced for (...) {if (condition) {return ...}} pattern by stream.filter(condition).findFirst() for clarity (and reusability)

todo:

• move controller logic into separate service
• update tests

fixes #842

[dennisvang]

The ClassFanOutComplexity error from checkstyle is an indication that the controller is doing too much.
This confirms the necessity of moving some of the controller logic into a separate service.