Implement prunerepo cron for PULP repositories

[praiskup]

We need to wait for PULP to fix this feature. Once that's done, we’ll need a reasonably optimized prune trigger on our side (there's no need to prune all existing repositories if there has been no traffic, or no new/removed packages).

[praiskup]

This is the feature we need to use: https://pulpproject.org/pulp_rpm/restapi/#tag/Rpm:-Prune

It wouldn't be wise to request repo pruning for all the repositories daily; we need to find some reasonable heuristic (e.g., prune those projects that appear to have recent build activity), nb. we have something like that in place for the backend storage).

[praiskup]

The bug report: pulp/pulp_rpm#4279