PROXY protocol is not configurable for "private" endpoint publishing strategy

Description of problem:

PROXY protocol cannot be enabled for the "Private" endpoint publishing strategy type.

Version-Release number of selected component (if applicable):

4.10.0+. PROXY protocol was made configurable for the "HostNetwork" and "NodePortService" endpoint publishing strategy types, but not for "Private", in this release.

How reproducible:

Always.

Steps to Reproduce:

1. Create an ingresscontroller with the "Private" endpoint publishing strategy type:

oc create -f - <<'EOF'
apiVersion: operator.openshift.io/v1
kind: IngressController
metadata:
  name: example
  namespace: openshift-ingress-operator
spec:
  domain: example.com
  endpointPublishingStrategy:
    type: Private
    private:
      protocol: PROXY
EOF

2. Check the ingresscontroller's status:

oc -n openshift-ingress-operator get ingresscontrollers/example -o 'jsonpath={.status.endpointPublishingStrategy}'

3. Check whether the resulting router deployment has PROXY protocol enabled.

oc -n openshift-ingress get deployments/router-example -o 'jsonpath={.spec.template.spec.containers[0].env[?(@.name=="ROUTER_USE_PROXY_PROTOCOL")]}'

Actual results:

The ingresscontroller is created:

ingresscontroller.operator.openshift.io/example created

The status shows that the spec.endpointPublishingStrategy.private.protocol setting was ignored:

{"private":{},"type":"Private"}

The deployment does not enable PROXY protocol; the oc get command prints no output.

Expected results:

The ingresscontroller's status should indicate that PROXY protocol is enabled:

{"private":{"protocol":"PROXY"},"type":"Private"}

The deployment should have PROXY protocol enabled:

{"name":"ROUTER_USE_PROXY_PROTOCOL","value":"true"}

Additional info:

This bug report duplicates https://bugzilla.redhat.com/show_bug.cgi?id=2104481 in order to facilitate backports.