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.
I'm a bit new to the facade patcher... what exactly are we testing here? That the above API client call Create() causes a successful request and response through the RPC layers? Or does the patcher skip that as well?

I'm having trouble seeing the point of this test. If something that uses the backup client API elsewhere (like cmd/juju) hit this code, that would make more sense to me, you'd be using the mocked out response to simulate an expected scenario.

In this case, I'm having a hard time seeing the point, though it could be my unfamiliarity with the approach or intended bigger picture.