Add method to manually interrupt the agent.

[martin-purplefish]

It would be nice to have more control over the agent from the outside.

One use case I have: The user hangs up while the agent is speaking. I would like to be able to manually interrupt the agent, so that I can log what the agent had already said on my backend.

Basically, in my participant_disconnected handler, I'd like to be able to say agent.interrupt() - which should interrupt uninterruptable speech - and then flush the final events so I can send them to my backend.