Create a Minio / S3 storage backend

[rustbomber]

Create a libunftp storage back-end implementation that stores files in Minio / S3

[hannesdejager]

Yup this would be a good one

[hannesdejager]

The implementor is of course free to publish their own crate instead of adding it to the codebase here. In that case it would be nice if its is name in accordance to the naming pattern i.e. unftp-sbe-* . This is to make unftp extentions easily findable.