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.
Does BackupClient need to be a sub-client for some other reason, or does it make sense to fit it in the pattern of the other API client "aspect" types in their own subpackage.

I could be convinced either way, just trying to understand why BackupClient needs to be special, if it does.