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 master.
• 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...

  ./cluster-setup/base/performance: OK
  ./cluster-setup/ci-cluster/performance: OK
  ./cluster-setup/ci-upgrade-test-cluster/performance: OK
  ./cluster-setup/manual-cluster/performance: OK
  ./cluster-setup/mcp-only-cluster/performance: OK
  ./cluster-setup/upgrade-test-cluster/performance: OK
  ./config/crd: OK
  ./config/default: OK
  ./config/manager: OK
  ./config/rbac: OK
  ./config/samples: OK

Summary: Checked 11 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/performance-addon-operators/actions/runs/18953897712/job/54125600143?pr=969 It's already running and passing! 🎉