add hook-as-on-config-vassal

Sometimes when a vassal is already on-demand and it is updated, it is helpful to be able to ask the emperor to run a hook.  For example, the hook can be used to log a message that the emperor has read the config change and is ready to accept requests for that vassal.

This is very similar to the hook-as-on-demand-vassal so the change is modeled after that.