context: add object store

Add method to set and get typed objects from the context. This can
be used to pass around context objects between modules without having
to register them.

src/pipewire/private.h

@@ -258,6 +258,8 @@ struct pw_context {
 	uint32_t n_support;		/**< number of support items */
 	struct pw_array factory_lib;	/**< mapping of factory_name regexp to library */
 
+	struct pw_array objects;	/**< objects */
+
 	struct pw_impl_client *current_client;	/**< client currently executing code in mainloop */
 
 	long sc_pagesize;