Add support for selectable fields

[clux]

CRD selectableFields kep

Basically, pick up on attrs:

  "x-kubernetes-selectable-fields": [
    { "fieldPath": "metadata.name"},
    { "fieldPath": "metadata.namespace"},
    { "fieldPath": "spec.myField"},
  ]

or whatever it actually is presented as in the schema, and use that to generate a #[kube] attribute matching the new support in kube-derive: kube-rs/kube#1605