Add kustomize validation check

[sebrandon1]

Similar to: openshift-kni/telco-reference#433

Changes:

• Adds a quick PR check that attempts to kustomize build appropriate files. Only runs against main.
• Adds a make path for make test-kustomize which runs the script. Users can run this locally as well as there are safeguards to ensure kustomize exists first.
• Skips YAMLs that might require external plugins. See EXCLUDED_DIRS.

Example run:

$ make test-kustomize 
hack/test-kustomize.sh
Checking all kustomization.yaml files can build successfully...

  ./config/crd: OK
  ./config/default: OK
  ./config/kind-ci: OK
  ./config/manager: OK
  ./config/manifests: OK
  ./config/networkpolicy: OK
  ./config/prometheus: OK
  ./config/rbac: OK
  ./config/samples: OK
  ./config/scorecard: OK

Summary: Checked 10 kustomization.yaml files, skipped 0 (require external plugins)
All kustomization files validated successfully!

[openshift-ci]

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[sebrandon1]

https://github.com/openshift-kni/numaresources-operator/actions/runs/18952979705/job/54122262347?pr=2398 It's already running and passing! 🎉

[ffromani]

The CI failure is legit, PTAL.

[ffromani]

Not sure how beneficial is this validation, but should not hurt either