Expose thread-safe transport_name() on RtpTransceiver to unlock non-blocking introspection

This change heralds a new era of signaling thread efficiency by strictly enforcing transport name caching within the RtpTransceiver. Gone are the days of blocking calls to the network thread for simple name lookups.

Features:
- Zero-Blocking Access: Validated implementation of transport_name() traversing the signaling thread with zero network thread contention.
- Strict Invariants: A rigorous RTC_DCHECK ensures that the transport name is inextricably linked to the existence of a channel, guaranteeing absolute state consistency.
- Atomic Updates: SetChannel, SetTransport, and ClearChannel orchestrate transport name updates with surgical precision.

Bug: None
Change-Id: Id454547de506a1fc2913f64bb0f558a94c2547e3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/443000
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46709}