Feature Suggestion: Implement S3 API

**is your feature request related to a problem? Please describe.**

When developing locally for S3, you usually need either a remote bucket or self host something like [Garage](https://garagehq.deuxfleurs.fr/) or MinIO, but they are usually pretty heavy and use their own specific format, they don't save files directly on disk iirc.

**Describe the idea / solution you'd like**

It would be awesome if even a subset of the S3 API could be implemented to use other S3 clients for accessing copyparty. Buckets might be simple directories with the respective files available.

**Describe any alternatives you've considered**
It might be possible to use rclone to run an s3 server that then forwards to copyparty's webdav or ftp server, but it would also be easier from a UX standpoint to offer it natively