Feature/krisp viva sdk support

[realgarik]

Add Krisp VIVA plugin for noise reduction and turn detection

Add KrispAudioInput wrapper for Krisp NC integration

Add AudioInput wrapper class to integrate Krisp noise cancellation into LiveKit Agents audio pipeline for human-to-bot conversations.

• New KrispAudioInput class wraps RoomIO audio input
• Add full example (krisp_agent_example.py) and minimal test
• Update README with integration documentation
• Audio flow: Room → RoomIO → KrispNC → VAD → STT → LLM