Introduce an Event when Annotations worker finished

### Describe the feature

Currently, there is no way to know when the Annotations worker has completed its task. This lack of notification can lead to inefficiencies in workflows that depend on the completion of annotation processing. To address this, we propose introducing an event that is triggered when the Annotations worker has finished its work.

### Use Case

We've encountered challenges in determining whether annotations are still running. The current workaround implemented in #4408 uses a recursive method call with a timeout, which is not an ideal solution. To address this issue more elegantly, we propose introducing an event that signals when the Annotations worker has finished its task.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [X] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### ACE version used

1.36.3