Enabling testing for new distribution

This document describes how to enable testing for a new distribution, or a new version of an existing distribution, in the suite of our projects. The process is quite complex and involves multiple repositories. The following steps should be followed in this specific order:

1. Add new distribution rpmrepo snapshots in osbuild/rpmrepo.
2. Trigger new repo snapshots.
3. Add distribution runners with image placeholders in osbuild/gitlab-ci-terraform.
4. Add support for the distribution in osbuild/image-builder.
5. Add support for the distribution in osbuild/ci-image-refresh-bot (internal only).
6. Run the ci-image-refresh-bot, so that it updates image placeholders in osbuild/gitlab-ci-terraform.
7. Add new repository definitions and distribution support (if needed) in osbuild/images.
8. Update unit test runners in GitHub actions (if needed) in osbuild/images.
9. Update .gitlab-ci.yml runners and Schutzfile snapshots for the new distribution and gitlab-ci-terraform ref in osbuild/osbuild to build osbuild RPMs for the new distribution.
10. Update the images ref in Schutzfile in osbuild/osbuild and add new distribution runners to test manifests in GitLab CI.
11. Update .gitlab-ci.yml runners and Schutzfile snapshots for the new distribution and the osbuild ref in osbuild/osbuild-composer.