MR webhook filtering by action type

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