feat(plugins-cartesia): Support 'volume' option in TTS constructor (Parity with Python SDK)

[Nikhil7174]

Your question

I am building a LiveKit agent using the Node.js SDK (@livekit/agents-plugin-cartesia). The default TTS volume is too quiet for some users, especially in a noisy background.

I noticed that the Python SDK (livekit-plugins-cartesia) supports a volume parameter directly in the TTS() constructor (e.g., volume: 1.5). However, the Node.js SDK does not seem to expose this option. Attempting to pass it via modelOptions or at the top level triggers TypeScript errors.