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.

spa/include/spa/support/plugin.h

@@ -37,11 +37,12 @@ struct spa_handle {
 #define SPA_VERSION_HANDLE	0
 	uint32_t version;
 
-	/* user_data that can be set by the application */
-	void *user_data;
 	/**
 	 * Get the interface provided by \a handle with \a type.
 	 *
+	 * \a interface is always a struct spa_interface but depending on
+	 * \a type, the struct might contain other information.
+	 *
 	 * \param handle a spa_handle
 	 * \param type the interface type
 	 * \param interface result to hold the interface.