feat: add optional kindFilter to fromCRD

[sini]

Summary

Adds an optional kindFilter argument to fromCRD — a list of CRD kind
names. When non-empty, only CustomResourceDefinitions whose
spec.names.kind is in the list are generated. The default [ ] keeps every
CRD found, so this is fully backwards compatible.

This mirrors what fromChartCRD already does with its crds argument, but for
the src-based fromCRD path.

Motivation

fromCRD's crds argument is a list of YAML files. When one of those files
is a multi-document stream — e.g. raw helm template --include-crds output, or
an upstream bundle that ships many CRDs in one file — you often want types for
only a subset of the kinds it contains, and no way to narrow that existed on
the src path. kindFilter provides it.

While supporting that, crd2jsonschema.py also learned to skip null
documents — empty or comment-only YAML docs, which yaml.safe_load_all
yields as None. These are common in helm template output and previously
made the generator raise; skipping them lets callers feed raw multi-document
streams to fromCRD directly.

Example

fromCRD {
  name = "cert-manager";
  src = ./crds;                                # a bundle with many CRDs
  crds = [ "crds.yaml" ];
  kindFilter = [ "Certificate" "Issuer" "ClusterIssuer" ];   # generate only these
}

Compatibility

• kindFilter defaults to [ ] → no filtering; output identical to today.
• The null-doc skip only avoids a crash on documents that would otherwise
  raise — it changes nothing for valid inputs.

Tests

Adds test_crd2jsonschema.py cases for kind filtering (hit + empty-filter
passthrough) and null-document skipping — nix run .#crd2jsonschemaTest.