Introducing Experimental GPT Assistant Agent in AutoGen (#616)

* add gpt assistant agent

* complete code

* Inherit class ConversableAgent

* format code

* add code comments

* add test case

* format code

* fix test

* format code

* Improve GPTAssistant

* Use OpenAIWrapper to create client
* Implement clear_history()
* Reply message formatting improvements
* Handle the case when content contains image files

* README update

* Fix doc string of methods

* add multiple conversations support

* Add GPT Assistant Agent into README

* fix test

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Co-authored-by: gagb <gagb@users.noreply.github.com>
Co-authored-by: Beibin Li <beibin79@gmail.com>