Ingest anonymous SSH URLs

Summary:

In swh/infra/add-forge-now-requests#282 (closed) we discovered a forge that only exposes anonymous SSH URLs to the repositories. There are working HTTPS URLs but the forge does not expose them. There may also be situations where a repository is only available over anonymous SSH. I attempted to use Save Code Now to save ssh://anonymous@got.gameoftrees.org/got.git and it was accepted, but it failed failed, so SCN will need modifications too.

See also: #5071 - this server has another forge that exposes the same set of repos, also only over anonymous SSH.

Plan:

• Implement mechanism
• Modify loaders
• Modify listers
• Modify Save Code Now
• docker: Run lister
• docker: Run loader
• Document lister
• Document loader
• Deploy on staging
• Call for public review
• Deploy on production
• Reopen affected AFNR: #250