implement an S3 backend

[clux]

No description provided.

[bencord0]

For local development, or for users who don't wish to use S3, we could use minio.

I think, that for projects in the open, it should be possible to fetch artifacts from a public S3 bucket, but gracefully deny publishing unless the user has the necessary access tokens to write to the S3 bucket.

For projects that don't want to have public artifacts, we should permit them to use arbitrary, S3 compatible, endpoints. With minio, this could be on a local laptop, but there are many other S3 implementations out there (I'm also a fan of OpenStack Swift for this purpose).

Minio is also available on the Docker Hub.