Webhook for Storage Sync Event in Label Studio

**Is your feature request related to a problem?**
I’m facing issues with the lack of automation options after syncing storage in Label Studio. Currently, there is no direct way to notify external systems or trigger further workflows when a storage sync operation completes in Label Studio. This missing webhook results in the need for frequent polling to check sync statuses, which increases the complexity of managing data workflows and makes it challenging to keep downstream applications in sync.

**Describe the solution you'd like**
I would like a webhook to be triggered whenever a storage sync is initiated or completed in Label Studio. Ideally, this webhook should provide details such as:

Storage ID or name
Sync status (e.g., started, completed, failed)
Timestamp of the sync event
Any error details if the sync fails
This would enable automatic workflows based on the sync status and allow downstream applications to receive real-time updates without constant polling.

**Describe alternatives you've considered**
**Polling the API**: I’ve tried using periodic API calls to check the sync status, but this approach is inefficient and not scalable, as it increases server load and API traffic.
**Custom Scripts**: I considered building a custom solution using Label Studio’s SDK, but this requires more development effort and is harder to integrate seamlessly with other tools that rely on event-based triggers.

**Additional context**
This feature could greatly benefit users who manage large volumes of external data in cloud storage. With this webhook, it would be possible to trigger automatic data processing steps immediately after data syncs, reducing manual intervention. It would also enable better tracking of data availability and faster reaction to sync failures in Label Studio workflows.