Add RFC 6062 TURN TCP relay support to TurnServer and ICE client

[CraziestPower]

Summary

The lightweight in-process TURN server (#1512, merged as #1513) only supports UDP relay. This issue tracks adding RFC 6062 (TURN Extensions for TCP Allocations) support to both the server and the client-side ICE stack.

This enables scenarios where UDP is blocked by firewalls and all media must be relayed over TCP.

Scope

Server-side (TurnServer)

• Allocate with RequestedTransport=TCP (0x06) creates a TCP relay listener instead of a UDP socket
• Connect (0x000a): server opens TCP connection to peer, returns ConnectionId
• ConnectionBind (0x000b): pairs a client data connection with a peer connection, starts bidirectional raw byte relay
• ConnectionAttemptIndication (0x001c): sent to client when a peer connects to the TCP relay listener
• Unsupported transport returns error 442

Client-side (ICE)

• IceServer.IceRelayProtocol property to request TCP relay
• RTCIceServer.X_ICERelayProtocol configuration option
• IceChecklistEntry TCP relay state machine (Connect → ConnectionBind → raw data)
• RtpIceChannel TCP relay send/receive paths
• IceTcpReceiver.RawMode for post-ConnectionBind passthrough

STUN layer

• Add missing enum values: ConnectSuccessResponse, ConnectErrorResponse, ConnectionBindSuccessResponse, ConnectionBindErrorResponse, ConnectionAttemptIndication
• Fix GetSTUNMessageTypeForId crash on unknown message type IDs

References

• RFC 6062 - TURN Extensions for TCP Allocations
• RFC 8835 Section 3.4 - Middlebox-Related Functions
• Related: WebRTC TURN over TCP needs adjust #1294, Enables ICE-TCP of TURN for WebRTC #1362 (ha-ves's draft client-side work)

[Forever-OnLine]

Hello, sorry to bother you. I would like to ask what this change means? In WebRTC, I noticed that the project ultimately uses UDP to transmit media streams. I want to switch to using TCP for transmission. Is this requirement now supported? If my application scenario is very simple, that is, it is applied in a simple internal network environment and does not involve public network constraints such as coturn, is it difficult for me to build a TCP-based transmission method by myself?

[sipsorcery]

This PR relates to a rudimentary TURN server implementation that's intended to be used for testigng only. It doesn't add TCP support for WebRTC connections.

There is already an IceTcpReceiver implementation but I've never tested it and it's not wired up everywhere, for example it doesn't allow remote TCP candidates.

So, while some preliminary work has been done on TCP support for RTP it's not fully wired up at this stage.