useAssistant: Add function to start a new thread/reset messages

[codinginflow]

Feature Description

There should be a way to clear the message history and start a brand new chat with an existing assistant. Since we can't clear a thread via the OpenAI Assistants API, I guess the correct way would be to start a new thread. To reset the threadId back to zero, in other words.

Use Case

Building a chat UI with a "clear chat" button.

Additional context

No response