Add callback for RtpSendParameters changes

The goal is to have a callback mechanism that allows rtp sender classes
to be notified when the RTP parameters change. That will allow the
senders to be able to maintain an accurate cache of the parameters that
can be queried without blocking on the signaling thread.

In practice this ends up covering the case where parameters change
during negotiation. In such a case the GetParameters()/SetParameters()
protection via RtpParameters::transaction_id doesn't protect against
overwriting those parameters (this situation is triggered by the
`EncodingParametersCodecRemovedByNegotiationVideo` test for example). By
using this callback, the sender can become aware of those changes too,
but that particular change is not a part of this CL.

Bug: webrtc:42222804
Change-Id: I1647fa9b175dfc38f10730e661d640fb6d668e6b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/441420
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46627}