This cl adds a forwarding header, a build target, and migrates headers in api/ to use it. Moving actual implementation, will follow, in https://webrtc-review.googlesource.com/c/src/+/265390. Bug: webrtc:12701 Change-Id: Id950725d7d054de8a83b3800b9c9a6437344de86 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/265660 Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org> Commit-Queue: Niels Moller <nisse@webrtc.org> Cr-Commit-Position: refs/heads/main@{#37200}