Add KittenTTS TTS plugin

[imdewan]

Summary

• add a LiveKit Agents TTS plugin for local KittenTTS synthesis
• expose model, voice, speed, clean_text, cache_dir, and backend options
• convert KittenTTS float audio chunks to 24 kHz mono PCM frames
• use the official KittenTTS 0.8.1 GitHub release wheel as the dependency

Tests

• python -m py_compile livekit-plugins/livekit-plugins-kittentts/livekit/plugins/kittentts/*.py tests/test_plugin_kittentts_tts.py
• python -m pytest tests/test_plugin_kittentts_tts.py -q

[CLAassistant]

All committers have signed the CLA.