Expose features currently provided by ModelController via capabilities usable by extensions

A server installs ServerService while an HC installs DomainModelControllerService, under different service names but both of which implement Service<ModelController>. To make it easier for subsystems that want ModelController access to work on both a server and an HC, we should create a capability with service type ModelController and have both processes use it.

Extensions may have a need to create an internal ModelControllerClient or to register notification handlers. Currently this is done by looking up the ModelController via its internal service name but:

1) ModelController exposes methods that should not be available to extensions
2) Accessing services should be via a capability.

So, we need to expose capabilities for these functions.