KnativeEventing CRD doesn't have proper openApi validation definition

[aliok]

Describe the bug
OpenAPI validation in KnativeEventing CRD is not good.
It doesn't provide anything useful.

$ kubectl explain knativeeventings.operator.knative.dev
KIND:     KnativeEventing
VERSION:  operator.knative.dev/v1alpha1

DESCRIPTION:
     <empty>

Expected behavior
Expected is something like KnativeServing example:

$ kubectl explain knativeserving
KIND:     KnativeServing
VERSION:  operator.knative.dev/v1alpha1

DESCRIPTION:
     Schema for the knativeservings API

FIELDS:
   apiVersion   <string>
     APIVersion defines the versioned schema of this representation of an
     object. Servers should convert recognized schemas to the latest internal
     value, and may reject unrecognized values. More info:
     https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#resources

   kind <string>
     Kind is a string value representing the REST resource this object
     represents. Servers may infer this from the endpoint the client submits
     requests to. Cannot be updated. In CamelCase. More info:
     https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#types-kinds

   metadata     <Object>
     Standard object's metadata. More info:
     https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#metadata

   spec <Object>
     Spec defines the desired state of KnativeServing

   status       <Object>
     Status defines the observed state of KnativeServing

To Reproduce

kubectl explain knativeeventings.operator.knative.dev

Knative release version
0.13.x

Additional context

[aliok]

/label good-first-issue

[knative-prow-robot]

@aliok: The label(s) /label good-first-issue cannot be applied. These labels are supported: proposal/0.11, proposal/0.12, proposal/0.13, proposal/0.14, proposal/0.15, proposal/0.16, proposal/0.17, proposal/0.18, proposal/0.19, channel/kafka, channel/natss, source/awssqs, source/camel, source/ceph, source/couchdb, source/github, source/kafka, source/prometheus

In response to this:

/label good-first-issue

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