Specify files for the reviewer to focus on in the code review
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### Proposal
From https://gitlab.com/gitlab-org/gitlab/-/issues/430123#note_1976603830
> In https://gitlab.com/groups/gitlab-org/-/epics/12878 we've built a new reviewer assignment that helps users focus on selecting the right reviewer based on approval rules. However, we [didn't go beyond assignment](https://gitlab.com/gitlab-org/gitlab/-/issues/441689#note_1775054231 "[Design] Incorporate approval rules when assigning reviewers to a merge request") and back into the sidebar and state of the MR. It would be good as you're thinking about the review flow pieces, how we could surface "WHAT" a reviewer has been asked to review.
There are levels to this from an author specifying which files they want focus on from a reviewer to using CODEOWNERS to automatically highlight files that match that reviewers assignment.
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