- Add SyncBlob method to sync service and on-demand interface
- Integrate blob sync into GetBlob API handler
- Use regclient to fetch blobs from upstream
- Automatically trigger sync when blob not found locally
- Handle sync errors gracefully with retry to local storage
This implements the core requirement from PR #3733 discussion:
check if blob exists, if not, sync from upstream on-demand.
Co-authored-by: rchincha <45800463+rchincha@users.noreply.github.com>