Add support for domains

[lubosmj]

Domains is an optional feature that allows multi-tenancy.

In the Pulp Container plugin, we need to deal with the fact that we have a single registry (a single source of truth, a remote server) that can be accessed by anyone. An open question is whether we should have a different namespace per each domain when serving the content or not. This would induce the requirement to reconsider redirects. Usually, CONTENT_PATH_PREFIX is a subject to change when dealing with domains' scopes.

https://docs.pulpproject.org/pulpcore/workflows/domains-multi-tenancy.html
https://docs.pulpproject.org/pulpcore/plugin_dev/plugin-writer/concepts/domains/domains_compatibility.html

Plugins adding domains support:
pulp/pulp_rpm#3106
pulp/pulp_ostree#346

[lubosmj]

Right now, we are issuing redirects, like this:

• v2/repo/manifests/bla -> pulp/content/manifests/id/

What would we like to implement?

1. v2/repo/manifests/bla -> pulp/default/content/manifests/id/ <----- bad, we do not know the domain ID from api-app
2. v2/default/repo/manifests/bla -> pulp/default/content/manifests/id/ <----- bad, we are breaking the standardized Registry API
3. No domains for pulp-container? Partial exclusion from the pulpcore's domain facility?

Needs more thinking...

[lubosmj]

The expansion to the domains' path will need to be identified on the pull level:

podman pull pulp.example.com/ipanova/busybox (no specified domain/namespace) -> podman pull pulp.example.com/default/ipanova/busybox

podman pull pulp.example.com/redhat/ipanova/busybox -> podman pull pulp.example.com/redhat/ipanova/busybox

The content-app path will be then explicitly retrieved from the API, like pulp/redhat/manifests/blabla.