TURN Client

[sipsorcery]

Add a new TURN client for allocating relay endpoints on TURN servers.

The TURN client has been wired up to work with the SIP media sessions. Wiring up for WebRTC is a TODO.

[Copilot]

Pull Request Overview

This PR introduces TURN client functionality to enable relay endpoints on TURN servers for SIP media sessions. The implementation provides a comprehensive TURN client that handles allocation requests, permission creation, and automatic refresh operations to maintain relay connections.

Key changes:

• Added a complete TURN client implementation with automatic allocation and renewal
• Integrated TURN support into SIP media sessions with relay endpoint management
• Refactored RTP channel architecture to support STUN/TURN message handling

Reviewed Changes

Copilot reviewed 28 out of 33 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/net/TURN/TurnClient.cs	Complete TURN client implementation handling allocate, refresh, and permission requests
src/net/TURN/TurnClientExtensions.cs	Extension methods for MediaStream to enable TURN relay functionality
src/net/TURN/TurnRelayEndPoint.cs	Data structure representing TURN relay endpoint information
src/net/RTP/RTPChannel.cs	Enhanced RTP channel with STUN message detection and relay sending capabilities
src/net/RTP/Streams/MediaStream.cs	Added TURN relay endpoint support and relay-aware RTP packet sending
src/net/RTP/UdpReceiver.cs	Extracted UDP receiver functionality from RTPChannel for better modularity
examples/sipcmdline/Program.cs	Updated command-line example to demonstrate TURN functionality
src/net/WebRTC/IRTCPeerConnection.cs	Added RTCIceServer parsing utility method
src/net/ICE/IceServer.cs	Enhanced IceServer with parsing capabilities and URI property

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