interface: add an interface struct

The interface struct has the type,version and methods of the
interface.
Make spa interfaces extend from spa_interface and make a
separate structure for the methods.
Pass a generic void* as the first argument of methods, like
we don in PipeWire.
Bundle the methods + implementation in a versioned inteface
and use that to invoke methods. This way we can do version
checks on the methods.
Make resource and proxy interfaces that we can can call. We
can then make the core interfaces independent on proxy/resource and
hide them in the lower layers.
Add add_listener method to methods of core interfaces, just
like SPA.

pipewire-pulseaudio

@@ -1 +1 @@
-Subproject commit a1a5ae1d6859b7ef2d55678e36fa5c9d458b1c65
+Subproject commit 2057d7955ce73db0e9cd16d38bbaf8c05b06fb80