Add review request button with optional message and category for reviewer

Problem

MR authors repeatedly write messages like this one to communicate their request for reviews -

" Could you please review this MR?" or " 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. " 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.