Prefer TURN candidates only

Description

When iceTransportPolicy is set to "relay" in the RTCConfiguration, the ICE agent will only gather relay (TURN) candidates. No host or server-reflexive (STUN) candidates are collected.

What do we do here?

We extract iceTransportPolicy from the peer configuration (from the logs). When it is "relay", we mark it as an observation.

Notes

• Using relay-only can improve privacy (hiding local IPs) but requires a working TURN server and may add latency
• With iceTransportPolicy: "relay", the absence of srflx and host candidates is expected and does not indicate a connectivity issue
• A prflx (peer reflexive) candidate can still be discovered from the remote end (e.g. via ICE); when using relay policy, such a candidate should point to a TURN server