chore: add backstage catalog entry for cern-magnum

Created a sample backstage catalog for cern-magnum. (despite looking like it) this is not a kubernetes CRD, which is why there is no namespace on metadata and there are some unfamiliar looking fields.

full spec gets defined here.

Once other components have their catalog entries created this will need to be updated to add it's dependents (i.e. the cern-owned helm charts we manage for example). I realise this is probably a bit early but it would be useful to have a small number of our services with these catalog entries so I can give a decent demo.