Batch SetupMediaChannel blocking calls in rtp receiver classes.

The current implementation of media channel setup triggers individual
blocking calls to the worker thread for every transceiver during state
updates. When applying remote descriptions with multiple transceivers,
this results in frequent thread context switching and increased
signaling thread latency.

Modify RtpReceiverInternal and its implementations to return a deferred
task using absl::AnyInvocable instead of executing the setup
immediately. This allows the SdpOfferAnswerHandler to collect setup
requirements across all transceivers and execute them in a single
batched blocking call on the worker thread.

Key modifications:
- Update SetupMediaChannel and SetupUnsignaledMediaChannel to return
  a callable task.
- Refactor AudioRtpReceiver and VideoRtpReceiver to package their
  worker-thread logic into returnable objects.
- Implement task collection and batch execution logic within the
  SdpOfferAnswerHandler.
- Update associated mocks and unit tests to accommodate the new
  asynchronous execution pattern.

(using No-try because of `webrtc_linux_chromium` timeout issues)

Bug: webrtc:42222804
No-try: true
Change-Id: I4b4d114e3f750fd41eaa4ed29e5c8ee6976eca39
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/447063
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46865}