Negotiate receiver reference time (RFC 3611 rtcp-xr:rcvr-rtt) in SDP

Add SDP support for negotiating non-sender RTT (RRTR/DLRR). A single
MediaContentDescription::receive_non_sender_rtt flag is the source of
truth; webrtc_sdp.cc is the only place that translates between the wire
forms and the flag:

- Parse: set the flag from either the standard a=rtcp-xr:rcvr-rtt or the
  legacy non-standard a=rtcp-fb:<pt> rrtr.
- Generate: when the flag is set, emit BOTH forms (rtcp-xr plus, for
  interim backward-compat, the legacy rtcp-fb rrtr per codec).

Advertised in offers/answers gated by the
WebRTC-RtcpXrReceiverReferenceTime field trial (default on, IsDisabled
to roll back). This CL only adds negotiation; enabling RRTR from the
flag is the stacked follow-up. When the legacy rtcp-fb form is dropped
later, only the parser/generator need change.

Bug: webrtc:516205747
Change-Id: Id7ad1e9b550bdfa372ddcf3dfdb0298310d407eb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/479240
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48051}