Separate transceiver references from the stats info struct

Extract the transceiver and receivers members from the
RtpTransceiverStatsInfo struct into a new TransceiverReferences
struct.

The RtpTransceiverStatsInfo structure is used to transport statistics
data between the signaling, worker, and network threads. Because the
transceiver and receiver objects are signaling-thread-specific, they
previously had to be manually cleared from the struct before it was
posted to the network thread to avoid cross-thread access.

Separating these references into a distinct container makes the data
ownership and thread boundaries more explicit. This refactoring
removes the need for manual pointer nullification and reduces the
risk of accidental cross-thread access during the statistics
gathering process.

Bug: none
Change-Id: I29a5fe77074a139ee9437dbca52b5450c51fa673
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/449040
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46906}