Negotiate use of RTCP loss notification feedback (LNTF)

When the LossNotifications field trial is in effect, LNTF should
be offered/accepted in the SDP message, not assumed to be configured
on both sides equally.

Bug: webrtc:10662
Change-Id: Ibd827779bd301821cbb4196857f6baebfc9e7dc2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/138079
Commit-Queue: Elad Alon <eladalon@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28056}

api/rtp_parameters.h

@@ -50,6 +50,7 @@ enum class FecMechanism {
 // Used in RtcpFeedback struct.
 enum class RtcpFeedbackType {
   CCM,
+  LNTF,  // "goog-lntf"
   NACK,
   REMB,  // "goog-remb"
   TRANSPORT_CC,