Support default hosts.toml configuration

[jess-sol]

Add support for an optional default registry hosts config, should no other hosts config match. Discussed in #7597

[k8s-ci-robot]

Hi @jess-sol. Thanks for your PR.

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[kzys]

Please update https://github.com/containerd/containerd/blob/main/docs/hosts.md as well.

[dmcgowan]

The change looks good. If we are going to build it into the default directory path logic then it could be ideal to choose a default directory which isn't also a valid domain name, such as _default. We try to avoid any possibility of ambiguity with edge cases when possible.

[jess-sol]

Yeah that's a good point, I hadn't really considered bare hostnames when I named it; updated!