Mechanism for disabling creating annotations and displaying highlights on the host frame for enable-annotations iframe use cases

Whenever the `VitalSourceInjector` injector is used, a `Guest` is not instantiated in the Host window. As a result, annotating is only possible in the `VitalSource` viewer even though Hypothesis is loaded in the parent frame as well.

Whenever the `HypothesisInjector` is used, [annotations are always enabled](https://github.com/hypothesis/client/blob/main/src/annotator/index.ts#L139) in the Host window. It seems there are valid use cases where this is simply not necessary, for example an LMS UI loading a reader type viewer in an iframe.

Perhaps a configuration property can be introduced that determines whether the `Guest` gets initialised?