Initial commit of GstTxFeedback
For TWCC (Transport-Wide Congestion Control) the stats you generate greatly improve with more accurate "sending-times", meaning the time a buffer left the GStreamer pipeline for the network.
This is a suggestion on how a generic mechanism could look for sinks (and other beasts) to communicate back upstream when a buffer was processed, that falls outside the domain of what you could know/assume just using PTS.