Additional WebHook event on Merge Requests when it has conflicts
<!--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=214372) </details> <!--IssueSummary end--> I'm currently using WebHooks to update a service I wrote to serve as ToDo list for my fellow devs, which gives them a quick overview of both their own merge requests that need attention (because they have open discussions or cannot be merged) as well as merge request they're supposed to look at (being set as Assignee). It mostly works, but there are two events that really make it difficult keeping the page up2date: 1. Pushing changes to the upstream branch that cause conflicts (usually after merging a _different_ merge request; causing the `has_conflicts` flag to flip from `false` to `true`) does not poke the WebHook. Basically like the email notification, except as a Webhook event.\ I got an item on my personal ToDo list (that I didn't get around to try yet) to use the Merge Request event that says "merged" or alternatively a Push event and check all _other_ open merge requests that target the branch just being updated; but I'm worried that it might take too long if the number or merge requests goes up; as well as the `has_conflicts` flag not being up2date yet. 2. ~~Marking a discussion as solved without adding a comment does not poke the WebHook (not with a Merge Request event, not with a Note event).\ This by itself isn't too important; but combined with the fact that it _might_ be the last open conversation (causing the `blocking_discussions_resolved` flag to flip from `false` to `true`) being resolved leaves my external page outdated.~~\ See #26234 for that. Adding both as trigger, preferably to the "Merge Request" event type (or at least the "Note" event type for the 2nd item) would really help with those tasks.
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