Introduce StreamProducer

Introduce a new gstreamer-utils crate that implements a StreamProducer structure that allows "producing" pipeline (Producers) pushing their data to 0 or several "consuming" pipelines. The Producer needs to push their data to an appsink which the StreamProducer controls and the "consumer" pipelines need to have an AppSrc which the same StreamProducer controls.

It allows similare behavior as a tee element but with a simpler to handle addition and removal of "consumers" as well as a total decoupling of the various consumer, both between each others and the producer pipeline.

This has simply been extracted from webrtcsink

ci: Re-implement the gi-checker in python to cleanup and ignore the gstreamer-streamproducer crate