Support additional notify events agent-turn-start

What feature would you like to see?

Right now the notify system only supports 'agent-turn-complete' as an event. https://github.com/openai/codex/blob/main/docs/config.md#notify

To support additional features (like indicating when codex is working), codex should support 'agent-turn-start' or a similar event.

Additional information

_No response_