Issue merge webhooks

### Problem Statement

Currently webhooks are fired only for a limited set of events:

```
action
type: string
description: can be created, resolved, assigned, or archived
```

There is another operation that is quite common but missing in this list: `merge`.

We use a custom service to synchronize issues/tasks between Sentry and Asana and merge operation on the Sentry side often breaks the linking on the Asana side. We would like to be able to detect the `merge` event and react accordingly.

### Solution Brainstorm

_No response_

### Product Area

APIs