Add async test (microsoft#520)

* Add example nb for async funcs

* Add a notebook based test for async function calls

* Update nb

* Update nb

* Remove duplicate code

* Rename func for consistency

* Fix bug

* Add intro text for cmd cell 4

* Add a short comment on await

* Update agentchat_function_call_async.ipynb

Minor typo

* Add link to nb

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Co-authored-by: Joshua Kim <[email protected]>
Co-authored-by: Qingyun Wu <[email protected]>