We need to provide additional functionality to our stakeholders that is
missing from GitLab's CE offering.

GitLab has no formal plugin APIs, and upstream seems [against the idea of
providing them][plugins]. GitLab's main service is built on Rails, however,
and Rails applications may load any number of [Rails Engines][engines] that
can acts as a subsystem of the main application, defining their own
initializers, routes, models, controllers, views, migrations, tasks, etc.

This project shows how we might inject our own `Gemfile` into GitLab's Rails
initialization process to load our own engines (or any gem really) that can
provide a path to extending GitLab in a standardized/module way.

Bug: T337570