As a user, I want to be able to list the Published Artifacts for a Publication

[daviddavis]

We had reports that one of the InRelease (a repo metadata file) for an apt repo was empty so we wanted to look through the PublishedArtifacts for the Publication to confirm if the CDN was serving the file or if the file was empty in Pulp. We had already republished the repo to fix the problem so we no longer could access the published artifact through pulp-content. Eventually we resorted to using the pulp shell to run PublishedArtifact.objects.filter(publication__pk="...", relative_path__contains="InRelease").

Ideally, we'd have a way to list/filter PublishedArtifacts without having to go to the pulp shell. I'd propose maybe a published artifact list endpoint. Ideally it would have fields like publication, digests, size, relative_path and users could filter by those fields.