CI: Please provide virtual machines via the Custom executor

The last upstream release, 12.1, ships a new Custom executor, which description reads:

GitLab Runner provides the Custom executor for environments that it doesn’t support natively, for example, Podman or Libvirt.

This gives you the control to create your own executor by configuring GitLab Runner to use some executable to provision, run, and clean up your environment.

Just to give two examples, why this might be benefical: This would allow to run autopkgtest within different environments, like different init systems, via rebooting the VM in between the tests. It would also allow run autopkgtest in an apparmor confined environment, to test profiles before doing the actual, possibly breaking, upload.

I'm happy to contribute code to enable this feature.