Add option to configure the Container Registry notifications

📻 Context

We are building more features around the GitLab Container Registry.

Several of these require to do something after a container image has been pushed. The main challenge here is that $ docker clients will push the image directly to the Container Registry backend without interacting back with the Rails backend.

One way we found to cover this need is using the Container Registry notifications. In very short words, the Container Registry can be configured to call a specific url as a callback after an operation happened, like an image push.

Rails has such endpoint and is ready to get those events.

However, these Container Registry notifications are disabled by default and needs configuration. This MR introduces the notifications_enabled boolean field under the registry to help with this configuration.

This way, developer don't need to recall which Container Registry configuration section has to be set up for notifications. It's all covered by the config templates.

🔬 What does this merge request do and why?

• Adds a notifications_enabled field under the registry key of the gdk.yml file.
• When enabled, configure a proper notifications section in the Container Registry config file.
• Update the gitlab.yml to have the correct shared secret string.
• Update the related specs.
• Update the related documentation.

⚗ How to set up and validate locally

Add the notifications_enabled field to gdk.yml:

registry:
  ...
  notifications_enabled: true

$ gdk reconfigure

Check registry/config.yml and you should see this part:

notifications:
  endpoints:
    - name: gitlab-rails
      url: http://<rails host and port>/api/v4/container_registry_event/events
      headers:
        Authorization: [notifications_secret]
      timeout: 500ms
      threshold: 5
      backoff: 1s

🎯 Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

🏁 Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.