Allow to negotiate dependency descriptor rtp header extension

Without exposing it in capabilities:
this extension is not stable enough to expose it by default,
but already in working state so with munge sdp can be experimented with.

Bug: webrtc:10342
Change-Id: I6bac123325a90431e4769e86da79638869e36cfc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168961
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30645}