Add openai_client as property and update notebook. (#654)

* Initial commit

* Add code interpreter notebook.

* Update code interpreter notebook.

* Update notebook.

* Update agentchat_oai_code_interpreter.ipynb

* Update agentchat_oai_code_interpreter.ipynb

* Add openai_client as property.

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