gitlab-housekeeper: Add support for finding a reviewer and refactoring

What does this MR do and why?

This MR is composed of a few changes:

1. The main thing is to automatically assign a reviewer from the group that owns the code being changed
2. This prompted a larger refactor because we had to add more data to the Change struct. The struct was getting out of hand so we extracted it to a real ruby object that can be built in steps and can also represent some of the transformations we need to do with the data
3. Adding more behaviour to this Change object prompted us to "promote" this object as the primary way of passing data around in the gitlab-housekeeper gem. So we now pass change into the GitlabClient. We plan to use this as a way to reduce the number of arguments being passed around everywhere. If and when the Change object becomes too complex we likely might want to start extracting mixins and such but for now it seems this design helped simplify method calls

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.