Add support for Ubuntu comand-not-found (CNF) indices

Author: wilful (wilful)

Redmine Issue: 9239, https://pulp.plan.io/issues/9239

Hi! I trying create mirror for official ubuntu repository: http://archive.ubuntu.com/ubuntu/
After creation on my test Ubuntu server i see error:

/pulp/content/archive.ubuntu.com/dists/focal/main/cnf/Commands-amd64: 404  Not Found [IP: 172.24.160.15 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.

It content (CNF) have in source but not in my repository
I use verbatim publisher, on_demand policy

      "pulp_href": "/pulp/api/v3/remotes/deb/apt/25c10483-64e2-492a-ac30-6efb5bbe3683/",
      "pulp_created": "2021-08-13T13:58:19.364006Z",
      "name": "archive.ubuntu.com",
      "url": "http://archive.ubuntu.com/ubuntu/",
      "ca_cert": null,
      "client_cert": null,
      "tls_validation": true,
      "proxy_url": null,
      "pulp_labels": {},
      "pulp_last_updated": "2021-08-13T13:58:19.364027Z",
      "download_concurrency": null,
      "max_retries": null,
      "policy": "on_demand",
      "total_timeout": null,
      "connect_timeout": null,
      "sock_connect_timeout": null,
      "sock_read_timeout": null,
      "headers": null,
      "rate_limit": null,
      "distributions": "trusty focal",
      "components": null,
      "architectures": "amd64",
      "sync_sources": false,
      "sync_udebs": false,
      "sync_installer": false,
      "gpgkey": null,
      "ignore_missing_package_indices": false
    },

Can i add it is cnf folder?

No files or tests are named in the issue. Start by reproducing the missing Commands-amd64 request against an Ubuntu focal mirror, then trace the Debian index synchronization and publication paths; done means the CNF index is available in the published repository without the 404.