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

[clux]

This should not be too hard; selectable_fields entry sits right on the crd.spec.versions[] which we extract in main here.

Once the values are there, they should map to top-level #[kube(selectable = "the-json-path-value")] for each selectable field. Leaving this as a good first.