Associate RTX with preceding codec in redesign

In the payload type redesign path, associate RTX codecs with the
immediately preceding codec in the supported formats list.

- pc/typed_codec_vendor.cc: Update CollectVideoCodecConfigurations to
  associate RTX with the preceding codec. Add validation to reject
  RTX preceded by a resiliency codec. Skip generic RTX at the end
  of the formats list if auxiliary codecs are added anyway.
- pc/codec_vendor.cc: Use MatchesWithCodecRules and negotiate
  packetization when merging codecs.
- pc/media_session_unittest.cc: Update AddRtxCodec helper to insert
  RTX after its primary codec in tests. Adjust test expectations to
  match negotiated codec order (which appends new RTX).

Bug: webrtc:360058654
Change-Id: I667089b8eef502b1da10e4ad811f919dadef2e6d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/479180
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47915}