Add a proof-of-concept direct interface to CBNG.

Following on from the outage of cbng-web this morning, I wondered what it would take to just call CBNG directly. This change is complimented by the proof-of-concept implementation at https://gitlab.cern.ch/pelson/cbng-py, but without it everything should still work as before (or not if cbng-web is down).

This prototype moved over to a full-fledged webservice in https://gitlab.cern.ch/acc-co/devops/cbng/cbng-web. It is easy to stand-up a local equivalent service (we do so in the tests), so I think this MR has run its course unless we'd prefer to use a local cbng before we try using a cbng-web-service. For now I'm closing this one.

closed