Support Hetzner Object Storage

There are some compatibility issues with newer versions of boto3 and botocore, but things at least seem to work on bookworm without too much difficulty.

It's a little odd that we're using the AWS provider type rather than fleshing out HETZNER, but I think it makes sense here since Hetzner uses S3-style credentials for object storage and its own credentials for other things.

Part of #543 (closed).