Add pipeline trigger feature

This MR adds the ability to trigger renovate-bot runs for a repository as part of the repositories CI. This should simplify the usage of the automerge feature as well as allow better automated dependency pinning as soon as they are commited to the project.

The feature takes the variable CI_RENOVATE_REPO, which should be the repository name of the triggering repository to tell renovatebot which repository should be targeted.

This feature could also be used, to have an own repository to manage own schedules for renovate to run, without requiring access to the main renovatebot repository and/or annoying other projects.

It also provides a gitlab-ci template to include in subproject to simplify maintenance.