Implement support machinery for payload type allocation redesign.

This CL introduces the basic building blocks and support machinery
for the payload type allocation redesign, while maintaining stability and ensuring all existing tests pass.

Key changes:

- Enhanced SdpPayloadTypeSuggester and FakePayloadTypeSuggester to
  track payload type mappings across media sections and handle late
  assignments.
- Fixed a crash in CodecVendor::GetNegotiatedCodecsForOffer by making
  video RTX linking conditional on finding the associated primary
  codec.
- Ensured pre-assigned codec IDs are registered with the suggester in
  AssignCodecIdsAndLinkRed to prevent collisions.
- Added comprehensive unit tests in
  pc/codec_vendor_redesign_unittest.cc
  and pc/typed_codec_vendor_unittest.cc (some video-specific
  redesign tests are disabled pending full implementation).

Verified with rtc_unittests, rtc_p2p_unittests, rtc_pc_unittests,
and peerconnection_unittests.

Bug: webrtc:42225436
Change-Id: I0050315115ae38a86901d215c63c1af82ac66e3f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/470881
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47683}