Backup S3-compatible bucket using Rustic

[Wouter0100]

It would be really nice to, instead of a filesystem or stdin, backup a S3-compatible bucket using rustic.

I've tried various techniques (rclone mount for example) and then read that mount by rustic, but it's very inefficient in deciding what files are changed and results in a lot of API calls to S3. I think it should be able to do it more efficient.