Add support for Additional_instructions

[CakeCrusher]

additional_instructions seem to only work ephemerally, where they are not stored in the database but are instead just passed to the run executor for that session only.

Example use:

    response = openai_client.beta.threads.runs.create(
        thread_id=thread_id,
        assistant_id=assistant_id,
        additional_instructions="Always add 'coconut' as the second word in your response.",
    )