Update plugin API to allow new resources creation

[aufi]

Phase 1 of migtools/crane#415

Phase 1: crane-lib API Extension Summary

This phase updates crane-lib to allow transform plugins to generate entirely new Kubernetes resources. The implementation ensures full backward compatibility without the need for a V2 API.

Core Architecture Updates

• Extended API: Both PluginResponse and RunnerResponse now include a NewResources field to hold standard Kubernetes unstructured objects.
• Graceful Degradation: Relies on the JSON omitempty tag. Older plugins simply omit this field, which automatically resolves to an empty list without breaking the execution.
• Simplified Runner Logic: The runner iterates through plugins and aggregates any NewResources provided. It avoids complex version validation, relying solely on whether the new resources array contains items.

Unit Testing Coverage

• Generating single or multiple new resources.
• Replacing an original resource (whiteout) with a new one.
• Verifying backward compatibility with older plugins.
• Aggregating outputs from multiple plugins and handling explicitly empty lists.

More details on https://github.com/aufi/move-crane/blob/main/drafts/plugin-update-new-resource-plan.md#phase-1-crane-lib-api-extension-tasks-1-2-4

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