Add support for post install/update announcements

What does this merge request do and why?

This MR adds support for display messages to the user after a gdk install|update so as to better inform our users of changes. It's pretty basic right now, but we can expand to support more conditional rendering logic as needed.

Closes:

• #750 (closed)
• #1570 (closed)

How to set up and validate locally

1. Checkout this branch, ashmckenzie/support-post-install-update-messages

2. Run make reconfigure

3. You should see at the end of the output, the following message:

     $ make reconfigure
     --snip--
   
     ℹ️  Announcements support added
     --------------------------------------------------------------------------------
     Announcements can now be added under data/messages as YAML files that will be
     rendered to the user after a GDK install or update.
   
     See https://gitlab.com/gitlab-org/gitlab-development-kit/-/merge_requests/2879 for more details.
   
     > Reconfigured as of 2022-12-09 15:27:32. Took 65 second(s).

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.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.