Add support for --scalability_mode in video_loopback utility.

Bug: webrtc:443906251
Change-Id: I9c8bf12d6ce39a018ffdba867e925afe6a10d9bc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/413621
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45838}