Ability to configure a listener to receive management notifications

Provide a means to configure registration of a listener registration for management notifications.
Users would implement a new NotificationRegistrar interface and register their impl in the configuration, with a management resource created for each configured NotificationRegistrar.
The interface would be invoked by the kernel during add of the NotificationRegistrar resource (at boot or post-boot if it's added then.)
The method passes in a ref to a client that subsequently can be used to invoke management ops, and a registry users can use to register listeners for future management notifications.