feat(sarvam): add realtime STT plugin for Saaras v3

[dhruvladia-sarvam]

Introducing STTStreaming, a LiveKit Agents STT implementation for Sarvam's
saaras:v3-realtime WebSocket API.

• WebSocket streaming with fast/balanced/simulated stream types
• VAD and manual endpointing, with configurable VAD query params
• Maps Sarvam events to LiveKit speech events (interim/final transcripts,
  start/end of speech) with symmetric utterance commit ordering
• Sets SpeechData.end_time from utterance speech-end position so EOU
  transcription_delay is computed correctly
• Captures server request_id and session_id as raw tracing values
• Usage metrics, reconnect on option changes, structured INFO logging
• Shared PeriodicCollector helper in _utils.py
• Export STTStreaming from the Sarvam plugin package
• Add unit tests for URL construction, event mapping, timing, logging,
  usage metrics, and error handling