Retrieve OpenAIAgent from existing assistant with assistant_id or thread_id

[illusion77]

Like the implemented feature in llama-index python version

agent = OpenAIAssistantAgent.from_existing(
    assistant_id="asst_xxx",
    thread_id="thread_xxx", # thread_id: Optional[str] = None,
    tools=[query_engine_tool],
    verbose=True,
    run_retrieve_sleep_time=1.0,
)