net/webrtc: sink&src: add signal "request-encoded-filter"
The new "request-encoded-filter" signal is emitted when the encoder and related elements are added to the pipeline. When defined, the element returned by the signal is inserted between the encoder and the payloader.
The transformation can be reverted using the insertable streams API on the receiver side.
Tested with h264 and vp8 streams using an element which adds a byte-size stamp at the end of each frame.
Requesting this against the 0.10 branch for now since this feature is needed on this branch. I will port it to main afterwards.
Edited by François Laignel