Add support for OpenAI structured output feature.

[urarik]

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OpenAI supports structured output as a feature. BeanOutputConverter will be more robust and predictable if SpringAI leverages it.

Expected Behavior BeanOutputConverter uses OpenAI structured output feature when using OpenAI.

Current Behavior BeanOutputConverter asks output format as a portion of prompt.

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