Add merge_request_author to description template

What does this MR do and why?

Add merge_request_author field to description template, where it takes the merge request author and creates a user.name <user.email> template.

When backporting fixes, the mr author might push code on behalf of another user, example redhat/centos-stream/src/kernel/centos-stream-9!3895 (merged), which makes the co-authored-by template perfect for listing the involved authors of an merge request, but it still misses the committer("X authored X weeks ago and Y committed"), this merge request adds merge_request_author to cover that gap.

Screenshots or screen recordings

How to set up and validate locally

1. On project settings, update the merge_template to include %{merge_request_author};
2. On a merge request of the same project, click edit template;
3. check if the template was properly populated with the mr author.

Signed-off-by: Lucas Zampieri lzampier@redhat.com