Add basic notebook for gptassistant (#636)

* Refactor GPTAssistantAgent constructor to handle
instructions and overwrite_instructions flag

- Ensure that `system_message` is always consistent with `instructions`
- Ensure provided instructions are always used
- Add option to permanently modify the instructions of the assistant

* Improve default behavior

* Add a test; add method to delete assistant

* Add a new test for overwriting instructions

* Add test case for when no instructions are given for existing assistant

* Add pytest markers to test_gpt_assistant.py

* add test in workflow

* update

* fix test_client_stream

* comment out test_hierarchy_

* Add basic gptassistant notebook

- also improve logging in gpt assistant

* Update notebook/agentchat_oai_assistant_twoagents_basic.ipynb

Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>

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Co-authored-by: Chi Wang <wang.chi@microsoft.com>
Co-authored-by: kevin666aa <yrwu000627@gmail.com>
Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>