Add notification mechanism for executed change requests

### Is your feature request related to a problem? Please describe.

When a change request is executed, an `AUDIT_LOG_CREATED` webhook event is fired containing details about the new feature state, with a log of "New version published for feature: my_feature". This is exactly the same as if the feature state had been manually modified, and there is no way to correlate this change with the change request that caused it.

Change requests are by nature asynchronous, often with long lead times of several days. As such, it's expected to have a mechanism to get notified when they are executed.

### Describe the solution you'd like.

Provide a new audit log webhook event for change requests being executed, with a reference to the change request itself and the changes that it caused.

The webhook payload should only contain a reference to the new feature state (if any), and not the feature value itself.

### Describe alternatives you've considered

It might be easier to augment the existing "feature state updated" event with a reference to the change request that caused it, but this would couple feature state updates to change requests. It's possible that change requests in the future might change other things besides feature states, or have multiple changes in a single change request.

### Additional context

_No response_