Cleanup policy objects + background job for execution.

Context

As part of the work to deliver the MVC of cleanup policies for packages, we need to create the policy objects and background jobs for execution.

Proposal

1. Cleanup policy objects + background job for execution. Weight 2.
   • ⚠ same thing here, we need logged statistics to monitor the load (how many cleanup policies to execute) they are at any time.
   • Regarding the cleanup policy parameter, we need some accuracy. Even though the UI, present it as number of packages to keep, it might be a good idea to have it named n_package_files_to_keep in the model.
     • The reasoning is that at some point, we will have a parameter to keep the n most recent versions. If we use n_packages_to_keep, this can be confusing.
2. GraphQL queries for cleanup policy objects. Weight 1.
   • This could be with the MR above but GraphQL MRs for shiny new objects tend to be quite massive. This huge code change could make the MR above grow out of bounds (eg. this is too big for a review).

Related to #342436 (closed).