Allow to choose the author of a MR through the APIs
### Description
I'd like to be able to specify the `author` of a MR through the MR APIs, instead of having it inherited by the API token's proprietary
### Proposal
Add a `author_id` to `POST /projects/:id/merge_requests`
### Links / references
### Documentation blurb
#### Overview
In my company we leave to the CI to automagically create MR for every branch (more on that: https://rpadovani.com/open-mr-gitlab-ci)
Unfortunately, the Gitlab runner uses a token defined as secret variable, therefore all the merge request have the same author
#### Use cases
We would love to have as author who first pushed the branch.
Another possible way to go is that the runner uses a token valid for the user who pushed the branch, but there is no way to do that automatically, I think?
### Feature checklist
Make sure these are completed before closing the issue,
with a link to the relevant commit.
- [ ] [Feature assurance](https://about.gitlab.com/handbook/product/#feature-assurance)
- [ ] Documentation
- [ ] Added to [features.yml](https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/data/features.yml)
issue