Add `cloudsmith` to the `publish` backends

[dhirschfeld]

Automatically uploads to multiple backend types:

• prefix.dev (with optional sigstore attestation support)
• Anaconda.org
• S3-compatible storage (with automatic channel initialization and indexing)
• Local filesystem (with automatic channel initialization and indexing)
• Quetz servers
• JFrog Artifactory

Cloudsmith?

Originally posted by @dhirschfeld in #5678 (comment)

[ruben-arts]

Does that not work?

[wolfv]

We need to add some custom code for the publish subcommand, indeed. PRs would be very welcome :)

[dhirschfeld]

We need to add some custom code for the publish subcommand, indeed. PRs would be very welcome :)

It's probably something I can manage! Will try and get some time in the next week or so.