Allow to schedule "prune"

[jkellerer]

This is a request to add prune to the list of ScheduledSections.

At the moment check, backup and retention can be scheduled. However one of the heaviest tasks, prune is not available. While retention can do prune as well it isn't very practical for large repositories (e.g. in my case, it runs +1 hour for a 500G repo on ordinary HDDs).

While retention policies can differ per path and tag, prune should just be run once per-repo (and at a lower rate). A separate schedule option would allow to configure this easily.

[jkellerer]

Btw. I'm open to provide this as a PR if that is fine.

[creativeprojects]

Sure, go ahead of you're happy to 👍🏻 it should be very similar to the check one