"auth-proxy-set-headers" should accept a configmap name without namespace

[isihu]

What happened:
When specifying a configmap for extra headers to send to the auth-proxy living in the same namespace I couldn't reach my service anymore.

# works
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: hello-world
  namespace: foo
  annotations:
    nginx.ingress.kubernetes.io/auth-url: http://auth.default.svc.cluster.local:8080/get
    nginx.ingress.kubernetes.io/auth-proxy-set-headers: foo/bar

# doesn't work
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: hello-world
  namespace: foo
  annotations:
    nginx.ingress.kubernetes.io/auth-url: http://auth.default.svc.cluster.local:8080/get
    nginx.ingress.kubernetes.io/auth-proxy-set-headers: bar

What you expected to happen:
Nginx should be able to read the name of the configmap living in the same namespace as stated in the docs.

NGINX Ingress controller version

-------------------------------------------------------------------------------
NGINX Ingress controller
  Release:       v1.9.4
  Build:         846d251814a09d8a5d8d28e2e604bfc7749bcb49
  Repository:    https://github.com/kubernetes/ingress-nginx
  nginx version: nginx/1.21.6

-------------------------------------------------------------------------------

PR that fixes this issue
#12997

[k8s-ci-robot]

This issue is currently awaiting triage.

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Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[github-actions]

This is stale, but we won't close it automatically, just bare in mind the maintainers may be busy with other tasks and will reach your issue ASAP. If you have any question or request to prioritize this, please reach #ingress-nginx-dev on Kubernetes Slack.