Refactor GPTAssistantAgent (#632)

* 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_

---------

Co-authored-by: Chi Wang <wang.chi@microsoft.com>
Co-authored-by: kevin666aa <yrwu000627@gmail.com>