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.
Also adds 2 fixes for issues that were encountered testing.
This is a forward port from this MR.