Notification actions can now be activated with an activation token
containing platform data. So update the notification-spec to include
latest version of the specification, and expose the activation token
during an action activation.
Not to change the API, by modifying the type of NotifyActionCallback
it's just better to provide a function to fetch the activation token
during activation only, so that this can be retro-compatible.
Reference: https://gitlab.freedesktop.org/xdg/xdg-specs/-/commit/b9a4700