Improve Pulp repository signing key update process

Context

For Pulp, the repositories are not updated automatically when a new repository metadata signing key is updated. Thus, we need to manual do it via creating a new publication for every repository.

In gitlab-com/runbooks!10154 (merged), the runbook was updated with detailed steps to update and validate. However, the process, especially the script, is manual and has many limitations (mentioned in gitlab-com/runbooks!10154 (diffs)).

This issue is created to evaluate better approach to the Pulp repository signing key update process, either by improving the script, or implementing an automation.