rtptwcc: introduce and implement the GstTxFeedback interface
The idea is that for TWCC you want to know as precisely as possible when a buffer you are sending leaves your system
By using this method, sinks can then report back their most accurate estimate of when this actually happened.
The implementation is a new GstTxFeedback meta with an interface that can be implemented by anyone interested in this accurate transmit time.
Edited by Håvard Graff