This adds an API that allows users to generate Markdown changelogs,
using Git commit messages and Git trailers as the input. The API is
hidden behind a feature flag called "changelog_api", and is disabled by
default.

For more information, see merge request
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52116 and epic
https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/351.