middleware: Implement storage update hook
- RESTBase compatibility:
- For each cache miss we send purge events to invalidate edge
- Purge events are app specific
- This hook allows post update integration
- Users can register a function to run on each update
- Defaults to no-op
- This hook allows post update integration
Example: PCS will inform upstream cache to invalidate URLs
Bug: T319365
Edited by Jgiannelos