Breaking Change - Make `gitlab-runner-helper-images` an optional dependency of `gitlab-runner` packages

To address Fix GitLab Runner Docker Packaging + bare insta... (#28014 - closed), over in Move exported helper images into separate package (!5190 - merged) we moved the exported helper images out of the gitla-runner packages and into their own gitlab-runner-helper-images. To avoid making a breaking change, the former package has a hard dependency on the latter package. Ideally the dependency would be weak (e.g. recommends in some OS), but since that would constitute a breaking change, we couldn't do it with Move exported helper images into separate package (!5190 - merged).

We could do it in the %18.0 iteration though.

Additional notes and references

1. This issue is the implementation issue.
2. The related deprecation notice is Deprecation - Make `gitlab-runner-helper-images... (gitlab#517765 - closed) • Axel von Bertoldi • 17.9