This is mostly useful when using the adapter with audiotestsrc,
which supports an infinite number of channels. By specifying
a preference, we can limit it to the number of channels that we want.

* there is just one link, no need for a list
* warn if something goes wrong and don't assert

streams should be provided externally,
the user may choose to create any kind of stream

This won't work with WpSpaPod, obviously...

It's not well-defined; we'll come back to that later

This reverts commit 4e8ae293.

Needed when using audiotestsrc with an adapter

This is a simpler endpoint implementation that does not require
any node setup. It should be good to use with test nodes (fakesrc/fakesink),
V4L2 nodes, and maybe others in the future.