Add to @AutoConfigureStubRunner stubs option

[marcingrzejszczak]

You could provide in the annotation on the tests which stubs you would like to have downloaded

[dsyer]

The best way to do this is the refactor StubRunnerProperties and pull all the Stubs properties up into the top level (there's no need for them to be nested anyway). Then we can simply add @PropertyMapping("stubrunner") to the @AutoConfigureStubRunner annotation.