Refactoring changelog tool to support monorepo

[doggydogworld]

Overview

This is a small refactor to the changelog tool to support the new monorepo setup we have. The main features introduced are:

• Added the ability to search through private repositories for changelog information
• Now supports enterprise specific changelogs through a new changelog-enterprise marker
  • ie changelog-enterprise: Introducing a new enterprise only feature or bugfix
  • This allows differentiating between enterprise only changes & core changes in the monorepo

Alternatives considered

• Labels for differentiating enterprise only changes from core changes
  • It's likely that a new use-case for PRs that contain both types of changes will occur in the new monorepo. A label would necessitate that PRs must be split based on whether they are an enterprise-only change or core change which is confusing and limiting.