Add review request button with optional message and category for reviewer
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=300895) </details> <!--IssueSummary end--> # Problem MR authors repeatedly write messages like this one to communicate their request for reviews - "<reviewer> Could you please review this MR?" or "<reviewer> Could you please do a database/docs/maintainer review?". It would save some time and effectively communicate the request if we could structure this and make it a part of the product. # Proposal/feature request Add a section on the MR page with a button that allows an MR author or repo developer to request a review, with the option of adding in a message. It should send an email/notification with the message. If the user specified no message, the notification could use a default message e.g. "<author> has requested a <maintainer/docs/backend> review on their MR". The MR should also be automatically assigned to the reviewer. As a first iteration, we could start with an API endpoint that takes `reviewer`, `message` (optional), and `category` (optional) as params. The reviewer categories could be defined per repo or group, e.g. `maintainer`, `database`, `documentation`, `backend`, `tests`, `frontend`.
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