Manage client store callbacks directly in the StoreService (#4923)
Manage client store callbacks directly in the StoreService (#4923)

When the client registers a callback with a `ServiceStore`, forward that
callback to the StorageService for registration in the appropriate store
on the service side.

Removes the need for a single channel of callbacks between each service
store and binding context, and removes the need to manage proxy
callbacks on the client side.
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