Add the API client side for backups.

Note that backups has its own facade, so it isn't quite as simple as adding a new method on Client.
It's facilitates testing the client code that wraps the underlying RPC facade call without having to invoke the actual RPC infrastructure.  That includes preparing the RPC args and handling the RPC result.  Really that's all we should be testing in client tests.  Everything else should be tested elsewhere in the appropriate layers.