API: Support Subschemas & Definitions with $defs & $ref

Per the OpenAI docs:

https://platform.openai.com/docs/guides/structured-outputs/supported-schemas

The new standard supports Structured model outputs by defining object schemas using Pydantic & Zod models.  Enabling this in Venice.ai is essential for the creation of private GraphRAG systems.  

Please click the link above and read on:
“Definitions are supported

You can use definitions to define subschemas which are referenced throughout your schema.”

Currently it appears the Structured Responses provided by Venice

Requires flattened schemas with strict: true

This effectively kills Venice from being an llm provider in our application.