generic: expose inter plugin

This new plugin exposes two elements, intersink and intersrc. These act as wormholes for data in the same process and can be used to forward data from one pipeline to another.

The implementation makes use of gstreamer-utils' StreamProducer, and supports dynamically adding and removing consumers, before and after producers, and changing producer names while PLAYING, both on the sink and the src.

This initial implementation comes with a small demo, and a few tests.