Introduce a new `GET /projects/:id/repository/changelog` API endpoint to get release notes for a given commits range

As suggested in gitlab-org/quality/pipeline-common#8 (comment 736011526), it would be great to introduce a new GET /projects/:id/repository/changelog API endpoint that would just return the release notes (instead of adding them to the CHANGELOG file).

This would work the same as the existing POST /projects/:id/repository/changelog API endpoint, except that it would return the release notes in the response body.