Make it easier to add namespaced rpc APIs.

Add an additional argument to the base create_rpc_dispatcher() method.
When a manager wants to override create_rpc_dispatcher() to add more
callbacks in their own rpc namespaces, it can just call the parent class
with the additional APIs to include.

Change-Id: I9cba8b176b35f55ba9d71365d0a8bf25d2ae311f
Merge "Make it easier to add namespaced rpc APIs."