This is the first step in implementing custom codecs in SDP.
Bug: none
Change-Id: I7789478208a769eaefd58b410ae6f488c604594d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/348662
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tony Herre <herre@google.com>
Cr-Commit-Position: refs/heads/main@{#42171}