[Request] More options for Volume backends (SFTP, S3, etc.)

[Rajdave69]

This is a little similar to #23, but broader.

Currently a lot of different repository backends are supported (S3, R2, SFTP, rclone, etc). It would be nice too if these were supported as volume backends as well.

This allows the possibility of there being one central 'backup' server, which pulls data from other servers' drives (SFTP for example) and creates backups of them locally.

This, having one Zerobyte server, is a better approach compared to installing it on every physical device.