Add template to push images to product meta-project Artifact Registry

Add a template to be included alongside Auto-DevOps.gitlab-ci.yml which takes images created from the $CI_DEFAULT_BRANCH (and possibly a configurable list of other branches) and pushes them to the Artifiact Registry of the specific product's meta-project.

This will enable us to get rid of the pull/push Terraform hackery that is currently in our boilerplate.