Add background job to propagate writes to built_in_project_templates_enabled setting
To support https://gitlab.com/groups/gitlab-org/-/work_items/21356+, we want to add a cascading setting that can manage the feature on an instance and group level.
Following the [cascading settings framework](https://docs.gitlab.com/development/cascading_settings/), we need to ensure writes to the setting are propagated to child groups. This is done with a sidekiq job.
Example MRs:
- [Service](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/192148)
- [Worker](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/192302)
- [Update](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/193351)
issue