Feature request: allow raw backup to be managed by external script

Hi,

first of all thank you for your usefull tool.

I was wondering if it would be possible to let an external script manage raw backups, so we can manage all kind of backup styles ourself:
- aws s3 glacier deep_archive, with aws cli tool
- azure storage with az cli
etc...

Regards,

Gaétan QUENTIN

The issue does not identify any files, tests, or entry points. Start by locating how raw backups are managed, then define the external-script interface and document supported behavior; done means raw backups can be delegated to an external script for services such as AWS or Azure.

Written by the indexing model from the issue text.