Maas-billing: maas api: privilege escalation via forged http headers due to missing authentication

A flaw was found in the MaaS API. This vulnerability allows any pod within the cluster to bypass the Kuadrant AuthPolicy gateway by forging HTTP headers, specifically `X-MaaS-Username` and `X-MaaS-Group`, which are trusted verbatim. This lack of first-party authentication enables an attacker to gain unauthorized access and escalate privileges. The concrete consequences include the ability to mint Kubernetes ServiceAccount tokens in other tenants' namespaces, revoke API keys, and exfiltrate sensitive model access configuration.