MR webhook filtering by action type
<!--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=584655) </details> <!--IssueSummary end--> ### Problem to solve GitLab merge request webhooks currently trigger for all MR actions (create, update, merge, close, reopen, etc.) without the ability to filter by specific action types. This results in webhook consumers receiving events for all MR activities, even when they only need to respond to specific actions. ### Proposal Add action-type filtering to merge request webhook configurations. Users should be able to select which MR actions should trigger the webhook, such as: * `open` - When an MR is created * `update` - When an MR is updated * `merge` - When an MR is merged * `close` - When an MR is closed * `reopen` - When an MR is reopened * `approve` - When an MR is approved * And other relevant action types
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