Add a per-container description to registry info

[hellcp]

Is your feature request related to a problem? Please describe.
Needed for M0ses/cooverview#19

Describe the solution you'd like
A formatted description that can include info about various things contained within the containers. Editing this description without pushing an update to the containers would be much appreciated. This should be per-container, not per project or package.

[hennevogel]

Not sure what you are asking for. If you want to enrich the data you show in the registry. What keeps you from doing it? Why do you need something 'new' in the OBS?

[hellcp]

I don't actually know how I want it, but I need to have a description that is easy to modify by container maintainer for registry/v2/<name>/info.json eg. kubic/cilium, which is generated by OBS, so I'm hoping this issue helps me and you to actually figure out how to best implement just that.