feat: add mirror handling and OCI mirror type

[wolfv]

This is the start of the "mirror" implemnetation that would

• be able to handle multiple mirrors of a single channel
• reroute a channel to an "internal" mirror (for on-prem deployments)
• handle OCI channels to pull data from

[wolfv]

Looks like some pushed artifacts have the wrong mediaType / no mediaType: pthread-stubs (https://github.com/orgs/channel-mirrors/packages/container/conda-forge%2Fosx-arm64%2Fpthread-stubs/23610821?tag=0.4-h27ca646_1001)

[baszalmstra]

I left a bunch of comments.

• Would be nice if you can take a look at removing all unwraps or replacing them with expects.
• I think the API for the MirrorState contains lots of values that are unused. It would also be nice to type it a little more. There are some implicit expectations in there now, like that the URL is sort of rewritten, but this is completely unclear from comments and/or types.
• I think perhaps it would be nice to split this PR into two. One for the mirrors and one for OCI.

[wolfv]

TODOs:

• sort channel keys from long to short, so that both https://conda.anaconda.org and more specific https://conda.anaconda.org/conda-forge work.
• add a "manifest" cache for OCI mirrors in order to only download the manifest once (instead of three times for checking jlap, zstd and bz2 existence (this one is deferred)
• fix issues on the actual OCI registry

I think for this PR we shoudl fix the first item, but defer the other ones (and fix them in a little while when the OCI registry itself is fully functional).