ingress-nginx auth-proxy-set-headers nginx configuration injection

A security issue was discovered in ingress-nginx where the `nginx.ingress.kubernetes.io/auth-proxy-set-headers` Ingress annotation can be used to inject configuration into nginx. This can lead to arbitrary code execution in the context of the ingress-nginx controller, and disclosure of Secrets accessible to the controller. (Note that in the default installation, the controller can access all Secrets cluster-wide.)