Quay: mirror-registry: ssrf: repo-level mirror accepts external_reference without url validation

A flaw was found in Red Hat Quay's repository-level mirror configuration feature. The POST and PUT handlers in endpoints/api/mirror.py accept an external_reference parameter without SSRF validation, unlike the organization-level mirror handlers which apply validate_external_registry_url(). A repository administrator can supply a crafted hostname that causes the Quay mirror worker to make requests via Skopeo to internal network services, cloud metadata endpoints, or other resources not intended to be reachable from the Quay application.