Allow config to change default repo

[sondavidb]

What is the problem you're trying to solve

Stemmed from containerd/nerdctl#2843.

We had an issue open in nerdctl where a user wished to set a default repository when a domain name is not set, mentioning that this is configurable in docker. This value is currently hardcoded as docker.io. I would like this to be configurable.

Describe the solution you'd like

A configurable option in a config file to get this behavior.

Additional context

I would like to drive this PR, but I'd like a bit more clarity on a few problems I saw while attempting to gauge effort and feasibility for myself:

1. Per Ahikiro's comment on the above issue, this should probably be configurable in the hosts.toml file, providing the documentation where similar logic lives. Should a user wish to set a default repo, I'm imagining they put it in certs.d/_default/hosts.toml with something like the following:

default_repo = "https://example.com"

This is the bare minimum that I would like to work for registries with no authentication or other verification. Please let me know if this is the correct way of approaching this.

2. I see that containerd 2.0 has moved the reference folder to its own repository: https://github.com/distribution/reference. In this case, I'm unsure if it is possible to override the default as it now lives in its own library, and we do not have access to this defaultDomain variable. Pull on its own does not qualify the reference, and it seems to be the responsibility of the caller to qualify the reference before calling Pull(). I wonder if this would also necessitate a contribution to distribution/reference?