Migrate merge request review_requested/removed events

What does this MR do and why?

It's an implementation of GitHub Importer - Migrate merge request review_requested/removed events

Screenshots or screen recordings

migrate_mr_review_requested-removed_events

How to set up and validate locally

Create project via import from Github. The project should have a pull request (merge request) that has review_requested and review_request_removed events. After import make sure that the merge request has the same events with appropriate author and time.

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.