Remove network thread blocking call from CreateChannel

Refactor the CreateChannel method in RtpTransceiver to remove the inline
BlockingCall to the network thread and utilize ScopedOperationsBatcher
to handle network thread operations.

The batcher allows the network thread tasks—specifically setting the RTP
transport and retrieving the transport name—to be queued. A finalizer
task is then used to safely update the transport name on the signaling
thread once the network operations are complete.

Updates include:
* Modified RtpTransceiver::CreateChannel to accept a batcher reference.
* Updated SdpOfferAnswerHandler to initialize and execute network
  task batches during SDP negotiations.
* Adjusted unit tests to accommodate the batcher-based flow.

RenegotiateManyVideoTransceiversAndWatchAudioDelay comparison runs of
blocking calls (peak values) with and without this change:

Operation             this CL origin/main delta
ApplyLocalDescription   25	  30       -5
ApplyRemoteDescription  22	  32      -10
DoSetLocalDescription   26	  31       -5
DoSetRemoteDescription  22	  32      -10
Close                   22        23       -1

Bug: webrtc:42222804
Change-Id: Ic585aa9d01b838a0366735013d58b541a41efd0f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/463000
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47415}