Support s3 bulket as channel

[fecet]

Since we already support using files as custom channels, using S3 as a channel shouldn't be difficult either. This would provide convenience for many enterprise use cases and also reduce the load on the prefix server.

[wolfv]

Hi @fecet - yes, we would be happy to merge a PR to that effect in rattler. We already support the gcs:// and oci:// prefixes and could similarly support an s3:// prefix. We have a middleware that could do the necessary request signing. Just someone needs to implement the bits and pieces :) Probably there is a crate to do it, too!