Mage target to create Pulp CLI configuration

🚧 Note that this MR builds on Implement mage pulp:supportedOSVersions target (!6024 - merged), and only the very last commit is relevant!

This MR adds a mage target to create a pulp cli configuration. The relevant bits of the config are the base_url pointing to our pulp instance, username and password, all three of which are obtained from the environment. The advantage of clobbering the existing config added in Install Pulp CLI in CI container (gitlab-org/ci-cd/runner-tools/base-images!86 - merged) is the ability to override the base_url. And this IS an advantage because we have a dev instance of pulp to test against.

The plan is for this command to be run in the same CI job(s) that puch packages to pulp. That will come later.