Exposing a spa_dict is necessary to allow using native pipewire API
that deals with these properties.

The internal structure change avoids mem copies when we need to
return a spa_dict.

This commits also removes exposing internal info structures via the
properties mechanism. This needs more thinking...

We are going to use this in other object types as well

* make the registries available as dynamic interfaces of the core
* improve a bit the C API of WpObject & WpInterfaceImpl