Use ScopedOperationsBatcher for media description pushdown

Modify RtpTransceiver and SdpOfferAnswer to batch media description
updates using ScopedOperationsBatcher. This replaces the previous
approach where each transceiver update triggered a synchronous blocking
call to the worker thread.

Sequentially blocking the signaling thread for each transceiver during
SDP renegotiation can cause performance bottlenecks and audio
impairments, so the responsibility of "backing off" is now with the
ScopedOperationsBatcher class instead of doing individual blocking
calls. By batching these operations, the work on the worker thread is
consolidated in a best-effort manner, and state updates to the signaling
thread are handled via finalizer tasks once the batch completes.

Bug: webrtc:42222804, webrtc:42223005
Change-Id: If826fdd30c1340afb0d426d2a7d1dac864976055
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/458360
Reviewed-by: Markus Handell <handellm@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47301}