MR set to Auto-Merge generates a webhook event
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### Release notes
<!-- What is the problem and solution you're proposing? This content sets the overall vision for the feature and serves as the release notes that will populate in various places, including the [release post blog](https://about.gitlab.com/releases/categories/releases/) and [Gitlab project releases](https://gitlab.com/gitlab-org/gitlab/-/releases). " -->
When a Merge Request is set to Auto-merge, a webhook event is sent.
### Problem to solve
<!-- What is the user problem you are trying to solve with this issue? -->
Currently, webhook events are only sent when a MR is updated, closed or merged. Some users may wish to automate certain tasks when a developer sets a Merge Request to Auto-Merge (for example, send a notification, or handle failures in the auto merge process).
### Proposal
<!-- Use this section to explain the feature and how it will work. It can be helpful to add technical details, design proposals, and links to related epics or issues. -->
When a user the Set to Auto-Merge button, a webhook event is generated similar to the events generated when a MR is updated.
### Intended users
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Platform Engineers may benefit from this by being able to build more reactive automation that responds to user intentions. Software Developers may benefit from the automation which this makes possible
### Feature Usage Metrics
<!-- How are you going to track usage of this feature? Think about user behavior and their interaction with the product. What indicates someone is getting value from it?
Explore (../../doc/development/internal_analytics/internal_event_instrumentation/quick_start.md) for a guide.
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Track number of webhook events delivered with this type.
### Does this feature require an audit event?
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https://docs.gitlab.com/ee/administration/audit_events.html
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No
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