Knative-ingress-gateway changes are overwritten by operator

[jheyduk]

Describe the bug
I changed my configuration as described on https://knative.dev/docs/serving/using-a-tls-cert/
On next reconcile my changes are lost.

Expected behavior
The operator should not overwrite the settings.
Otherwise the documentation should extended how to make the changes persist using KnativeServing CRD

To Reproduce
Make some changes on Knative-ingress-gateway and wait for the next reconcile. The added -hosts-Section will disappear

Knative release version
v0.21

[houshengbo]

If you install knative serving with the operator, configure it with the operator.

In terms of Knative-ingress-gateway, you should configure it as the instructions here: https://knative.dev/docs/install/operator/configuring-serving-cr/#replace-the-default-istio-ingressgateway-service or https://knative.dev/docs/install/operator/configuring-serving-cr/#replace-the-knative-ingress-gateway-gateway

If you want to change the default content of knative-ingress-gateway, operator reconciling loop will revert it, because it loads the default knative-ingress-gateway each time and apply in the loop.

[houshengbo]

It also reminds me something similar in the old archive:
knative/serving-operator#258
knative/serving-operator#302

Time to revive.
The advanced k8s resource overlays looks charming, but it is still challenging for me to move on in that direction, because it will indicate knative operator has given up the effort to build a strong-typed API.
To support the configuration, I would rather accept the PR to override the servers section for gateway.