session-manager: fix protocol

Add separate types for interfaces and implementation
Fix some bugs in the protocol
Add some introspection support in cli

src/modules/module-session-manager/endpoint.c

@@ -286,6 +286,7 @@ int endpoint_init(struct endpoint *this,
 	const char *keys[] = {
 		PW_KEY_FACTORY_ID,
 		PW_KEY_CLIENT_ID,
+		PW_KEY_DEVICE_ID,
 		NULL
 	};