Update first packet callbacks to include SSRC information

Update the received packet callbacks to pass the SSRC of the incoming
packet. This allows RtpReceiver and RtpTransceiver to associate the
"first packet" notification with a specific media stream, providing
better context for state changes and down the line inform the signaling
thread of when an SSRC is signaled. That in turn will facilitate the
ability of being able to read the ssrc of receivers on the signaling
thread without a thread hop.

Additionally, rename SetFirstPacketReceivedCallback and
SetFirstPacketSentCallback to include the "_n" suffix, explicitly
indicating that these operations must run on the network thread.

Bug: none
Change-Id: Ib71058f17d1f4d506a7d46da1f012269b923b6b7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/449460
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47066}