Add AbslStringify for EcnMarking enum

To have centralized way of string (and log) representation of this enum.

Rewrite values as binary to stress they match bits as written in the IP
header.

Bug: webrtc:42225697
Change-Id: I8d8d725bdf09afa479c3470eb692c4a64e828739
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/442720
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46698}