Add implementation for endpoint `/projects/:id/issues/:issue_iid/closed_by`
See: https://docs.gitlab.com/ee/api/issues.html#list-merge-requests-that-will-close-issue-on-merge
I require this for a project I am working on. I implemented the code and I am attaching a patch file. Unfortunately it seems impossible to push new branches for me on this gitlab instance.
Alternatively you might consider the commit from gitlab.com I pushed here: https://gitlab.com/InstaffoOpenSource/rust/gitlab-rust/-/commit/aabac230e2984b8666aacd7c54c5d2c1a7ac3cae
0001-api-projects-add-issues-merge_requests_closing-endpo.patch