Add support for using the GitLab changelog API
This adds ReleaseTools::ChangelogCompiler, used for generating changelogs using GitLab's new changelog generation API. We are introducing this as a separate class and namespace as to not affect the existing code, allowing for an easier incremental roll-out of this new approach to generating changelogs.
This class doesn't reside in the ReleaseTools::Changelog namespace, and this is deliberate. When all projects have switched over, the various classes in this namespace are no longer needed. Having a namespace for a single class is overkill, so we simply place this new class directly in the ReleaseTools namespace.
See gitlab-com/gl-infra/delivery#1405 (closed) for more information.