Adds rust version of webrtc::RateTracker 

This includes some updates to how units FFI is exposed and removes the
previous bridge tests as now we have real tests for cxx bridges.

Bug: webrtc:416446214
Change-Id: I027e68b4521e14d5a1687dd0f0d03ed16a6a6964
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/470180
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Auto-Submit: Evan Shrubsole <eshr@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Reviewed-by: Victor Boivie <boivie@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47663}