feat: support Kustomization diff with ExternalArtifact

```
2025/11/20 13:41:37 Generating FluxCD diff for Kustomization confighub/default-reloader using actual FluxCD Builder
2025/11/20 13:41:37 Error generating FluxCD-style diff: failed to get source artifact: unsupported source kind: ExternalArtifact
```

when trying https://github.com/confighub/flux-bridge