recursive schemas not supported

The schema validator used by Kubernetes server and kubectl-validate does not support recursively defined schemas. This can be fixed but it would be a fairly big modification.

CRDs cannot be recursive, so this is not a concern for them. There may be some native types that have a recursive schema. One example of this is CustomResourceDefinition itself. n this single case, we workaround this by using the handwritten validation code for CRDs since they are so common.

In general, we should detect and block validation of recursive schemas.