Move ScopedOperationsBatcher to its own source files

Extract the ScopedOperationsBatcher class from sdp_offer_answer.cc into
a new, dedicated library module. This change improves code
organization by isolating the batching logic, which handles
synchronous task execution on the worker thread and optional
callback tasks on the calling thread.

As part of this refactoring:
* Created scoped_operations_batcher.h and .cc.
* Defined a new rtc_library target in pc/BUILD.gn.
* Added unit tests in scoped_operations_batcher_unittest.cc to
  verify task execution on both worker and signaling threads.
* Updated sdp_offer_answer.cc to include the new header.

Bug: webrtc:42222804
Change-Id: I090c1614917e83a3e3d9543991f1fa2dc4cf22f7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/454305
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47199}