Embed ProviderClient in ServiceClient.

This is something that's been bugging me for a long time. When I was doing the ServiceClient stuff, I didn't know that you could also embed structs as pointers, so I made the ProviderClient reference a named field, but it's much more convenient if it's not.

This is a quick touchup to make it embedded and a bunch of find-and-replace to make the references cleaner.