openai: add support for sending Structured Output request for chat completions api

[takeyamakenta]

https://openai.com/index/introducing-structured-outputs-in-the-api/

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[kei01234kei]

@tmc
Could u review this pr? I'm looking forward to using structured output in langchaingo!!

[tmc]

LGTM can you add an example showing use of this?

[lifejwang11]

LGTM can you add an example showing use of this?

i have a example of this , please check out it. #1098