Openai assistant function usage notebook (#639)

* add oai assistant function call

* polish

* add titles

* polish

* Update notebook/agentchat_oai_assistant_function_call.ipynb

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

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Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>
Co-authored-by: Chi Wang <wang.chi@microsoft.com>