Debian Package Manager MVC

We want to add Debian archive/repository support to our Artifact repository, to allow it to serve

Suggested MVC:

• Utilizes new repositories API endpoint: https://gitlab.com/gitlab-org/gitlab-ee/issues/5838
  • Only support a single repository with the name debian for now. Automatic layout.
• HTTP(S) based on GitLab's config (no ftp/sftp for now)
• Authentication
  • apt does support HTTP basic authentication, but we will need to see if it supports header based authentication which we utilize today with our API
  • If so, we should add support for PAT token or CI Job Token, since our API supports that today
• Storage
  • Local and Object Storage
• Downloading packages
  • Making packages available seems to be simple, apt just expects a specific layout and file formats for indices
• Uploading packages
  • Generally files get placed into a queue on disk, which are then processed
  • For the MVC we can consider two stages. First is a package/upload stage, which persists the package as an artifact. The second can be a process stage, which is triggered via an API which automatically adds the package to the desired archive. This stage could be a canonical CI job which does the index regeneration based on a CI image for now, to be built into GitLab later.

See #5835 (comment 414103286) for implementation plan and MR progress and #5835 (comment 463532286) for proposed model structure.