This PR removes the direct dependency of the IP allowlist from
CancelClosure, allowing for more scalable and flexible IP restrictions
and enabling the future use of Redis-based CancelMap storage.
Changes:
- Introduce a new BackendAuth async trait that retrieves the IP
allowlist through existing authentication methods;
- Improve cancellation error handling by instrument() async
cancel_sesion() rather than dropping it.
- Set and store IP allowlist for SCRAM Proxy to consistently perform IP
allowance check
Relates to #9660
fcfff72454