Require field trial parameter offer:true to create offer with RFC8888

To both offer and answer with RFC 8888 support, use field trial string "WebRTC-RFC8888CongestionControlFeedback/Enabled,offer:true/"

The purpose is to simplify rollout.
ie. first, an experiment with answering an offer with RFC 8888 can be rolled out. This should be a
noop. Later enabling sending an offer should enable using RFC8888.

Bug: webrtc:42225697
Change-Id: I2b4e52a9335303f2ef6a744027d83eb7f18e1edf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/404041
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45671}