OpenAI structured outputs support

[antoniomdk]

Feature Request

I've been working with typia.llm.schema for a while and it has been extremely helpful in generating JSON schemas to call LLMs from TS types. However, the new structured outputs API of OpenAI has some limitations in the type of schemas it can take.

In particular nullable is not been taken into account. So it'd be great if we could map types X | null to anyOf. Maybe introducing a new flag to the typia.llm.schema function.

Also, for types that don't extend from Record, we should mark [additionalProperties to false](https://platform.openai.com/docs/guides/structured-outputs/additionalproperties-false-must-always-be-set-in-objects).

I can contribute to this feature, but I may need some pointers for code references to start.