Add a basic stub client.

Hey.

It'd be nice to have a stub client maintained alongside for users (me :D) to use.

We have something like this locally already in our codebases, but maintaining it as public API inside the library would be helpful to ensure that its interface is maintained as changes are made.

I've included a very basic one. There's plenty missing but it does the basics, which is all we need, at least right now. If others are interested I'm sure there'd be more to add.