Add API for generating changelogs in GitLab

What does this MR do?

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 !52116 (merged) and epic gitlab-com/gl-infra&351 (closed).

This fixes #300224 (closed) as well.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content