Batch up blocking calls when creating media channels in RtpTransceiver

Refactor RtpTransceiver to support media channel and sender creation
asynchronously on the worker thread. Construction still can't be
considered complete until all the same steps have completed. This change
batches up multiple blocking calls into a single one to the worker
thread during transceiver initialization.

Key modifications include:
* Utilizing ScopedOperationsBatcher to gather channel and sender
  initialization tasks on the worker thread.
* Splitting transceiver setup into a worker thread task (for media
  creation) and a finalizer task on the signaling thread (to update
  internal state).
* Updating SdpOfferAnswerHandler and RtpTransmissionManager to pass the
  batcher through the transceiver association and creation workflows.
* Ensuring all transceivers are fully constructed before proceeding
  with channel updates during SDP negotiation.

Bug: webrtc:42222117
Change-Id: I572a4b5cf474df583a3e4dd9328d57bb1d51cab5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/463900
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47491}