assistant2: Add rudimentary chat functionality (#21178)

This PR adds in rudimentary functionality for sending messages to the
LLM in `assistant2`.

<img width="1079" alt="Screenshot 2024-11-25 at 1 49 11 PM"
src="https://github.com/user-attachments/assets/5accb749-c034-4fb2-bf55-3ae5bc9529ad">

Release Notes:

- N/A