Multi container registry support

[mxey]

for #56

Open issues:

• registry.k8s.io is hardcoded, no configuration
• no authentication for registries yet (should probably detect the auth URL from the registry response headers)
• cache directory does not account for registry: registry.k8s.io/kube-apiserver becomes cache/artifacts/oci/kube-apiserver

[mxey]

@andrew This already works for registry.k8s.io because it requires no authentication. I haven't added any tests or configuration yet, but is this a general direction you'd accept?

Also CONTRIBUTING.md didn't say whether you prefer multi-commit PRs or stacked PRs.

[mxey]

The auth parts of this might have to change depending on #186

[markkrj]

I found @kpfleming approach of making it configurable much better:
https://github.com/git-pkgs/proxy/pull/93/changes#diff-c7f43f17867c7240c4a26f92d204f0999bba302964d8f9abbc10e9103ffb066aR173

[mxey]

@markkrj Do you mean just the fact that it's configurable? As I said in my initial PR description, configuration is still missing. This is a prototype to gather feedback on the basic idea.

[andrew]

The /v2/<registry> virtual-registry approach looks like a good direction.

Two things worth settling early:

1. The registry alias should be part of the cache/database identity so identically named repositories from different registries cannot collide.
2. Authentication should be discovered from WWW-Authenticate and performed lazily on a cache miss. That should also let cached blobs remain available when the upstream auth service is down.

Otherwise, this looks like a useful prototype to continue developing.