New Tools: SMS pre-built tools

[Hormold]

Part of https://github.com/livekit/agents/pull/3944

SMS Tool - Many voice agents need to send text messages during calls (order confirmations, addresses, booking links, etc.). This tool provides a simple way to send SMS via Twilio, SignalWire, or Vonage. It can auto-detect the caller's phone number from the SIP session or use pre-configured recipients. The goal is to reduce boilerplate for this common use case while keeping it flexible enough for different workflows.