[POC] Expose retrieval SDG as a Data Designer recipe

[oliverholworthy]

Summary

POC consumer for the Data Designer recipe discovery interface proposed in NVIDIA-NeMo/DataDesigner#613.

This keeps retrieval SDG in the external plugins repository, but makes it discoverable and runnable from Data Designer when the package is installed:

data-designer recipes list --output json
data-designer recipes help retrieval-sdg
data-designer preview --recipe retrieval-sdg -- --input-dir ./docs

Design

• Registers retrieval-sdg in the data_designer.recipes entry point group.
• Adds a recipe module with load_config_builder(params) for execution.
• Adds build_typer_app() so Data Designer can inspect recipe-specific arguments and render help from the same Typer command shape used by the recipe.
• Reuses the existing retrieval SDG pipeline builder instead of moving application-specific workflow logic into DataDesigner core.
• Documents the recipe-based preview path in the plugin README.

Included Fix

This also changes EmbeddingDedupColumnConfig.get_column_emoji() to a static method. Data Designer calls column emoji helpers on the config class, so the previous instance method raised a TypeError during report generation.

Dependencies

This PR depends on the DataDesigner recipe discovery POC in NVIDIA-NeMo/DataDesigner#613, which itself builds on NVIDIA-NeMo/DataDesigner#612.