[POC] Expose retrieval SDG as a Data Designer recipe

[oliverholworthy]

Summary

This is a proof of concept showing how an application-level retrieval SDG pipeline can be exposed as an installable Data Designer recipe.

It registers a retrieval-sdg recipe entry point, adds a typed recipe config model, maps recipe config into the existing pipeline builder, and documents the run-recipe workflow.

Notes

• POC intended to pair with the DataDesigner recipe CLI PR.
• The recipe keeps pipeline-specific orchestration in the plugin package while exposing a reusable Data Designer entry point.

Testing

• data-designer recipes list
• data-designer recipes show retrieval-sdg

[oliverholworthy]

Closing this older plugin-side POC as superseded by #14.

The replacement PR targets the newer recipe discovery design from NVIDIA-NeMo/DataDesigner#613 and keeps retrieval SDG as an external consumer registered through the data_designer.recipes entry point group.