lib: spa-device: add wp_spa_device_set_managed_pending()

Allow marking WpSpaDevice object ids "pending", which means Props from any ObjectConfig events received for the ids are saved, if there is no associated object set yet.

When wp_spa_device_store_managed_object() is called, any pending Props are set on the managed object.

This is useful when nodes cannot be immediately created in the "create-object" signal handler. For example, in cases where the nodes are created asynchronously, e.g. by "module-loopback". In this case, although the nodes can be later associated with the WpSpaDevice, any ObjectConfig events received in the meantime are lost, so for example restoring saved Routes will race against async node creation. Using wp_spa_device_set_managed_pending() solves this race condition.