This allows configuring an `appsrc` before actually adding it as
consumer at a later time, and can be useful if the pipeline with the
source should be started earlier than adding it to a consumer.
Also use strongly-typed property API for the `appsrc` properties.
Introduce a new `gstreamer-utils` crate where we implement 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](https://github.com/centricular/webrtcsink/blob/main/plugins/src/webrtcsink/utils.rs)
