Add support for S3 backend

[pavelzw]

With s3 support coming to pixi (prefix-dev/pixi#2825) and already being in conda since a long time, it would be cool if we supported s3 backends in this tool as well

I don't have an immediate use for this, though. Just wanted to share this idea

[jaimergp]

Required changes would probably go in conda-forge-metadata first, to stream the metadata from the artifacts (or maybe conda-package-streaming?), and maybe we have some channel-specific bits to cover here too.

Will be easier once there's a public S3 bucket to test against.

[pavelzw]

Will be easier once there's a public S3 bucket to test against

actually, public s3 buckets are just like http servers so they should work out of the box 😅

[ytausch]

Good point, so we don't need to do anything here?

[ytausch]

How about authentication for private buckets?