knative: support ingressClass annotation

[ilackarms]

currently Gloo's Knative controller processes all ingresses.networking.internal.knative.dev, ignoring the user's custom ingress class defined here: https://github.com/knative/serving/blob/master/config/config-network.yaml#L89

With the addition of new conformance tests to knative (knative/serving#6305), Gloo should support filtering ingresses based on the networking.knative.dev/ingress.class annotation (with backwards-compatibility for users who do not currently rely on the annotation).

See https://github.com/knative/serving/blob/604bdd3afc450e9b74cb648bf4f2367898553296/pkg/apis/networking/register.go#L36 for definition in Knative

[ilackarms]

networking.knative.dev/ingress.class=gloo.ingress.networking.knative.dev is the expected annotation

[ilackarms]

in order to provide backwards compatibility, we need to provide a setting to toggle this behavior.

installing Knative will also require additional configuration step for users to configure knative's network-config to use gloo as its ingress provider

Gloo and Knative installation docs (as well as glooctl) will need to be updated in accordance with the new install step.

[ilackarms]

cc @kdorosh @marcogschmidt @rickducott