Introduce SetSsrcTask and batch SetSsrc in ApplyLocalDescription

The existing SetSsrc implementation in RtpSender required multiple
synchronous blocking calls to the worker thread. In scenarios with many
transceivers this was a significant performance bottleneck.

This change introduces SetSsrcTask, which allows SetSsrc operations to
be decomposed into asynchronous tasks. By using ScopedOperationsBatcher,
these tasks are now batched and executed efficiently on the worker
thread. The existing SetSsrc will be for PlanB only.

Key modifications:
* Added SetSsrcTask to the RtpSenderInternal interface to return a
  batchable task containing both worker and signaling thread logic.
* Updated SdpOfferAnswer::ApplyLocalDescription to use
  ScopedOperationsBatcher, reducing the number of thread context
  switches.
* Moved media channel configurations (frame encryptors,
  transformers, and selectors) into the batched worker task.
* Marked the legacy SetSsrc method as Plan B only.

Bug: webrtc:42222117
Change-Id: I49cfbeef4764d8423d678eec4f5c8017374e3187
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/465020
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47507}