Implement the `injectClient` method of Guest that is used by
integrations (eg. VitalSource) to inject the client into a chosen frame.

In the process the `HypothesisInjector` class has been extracted out of
the `CrossFrame` class, since it is unrelated to the rest of the
functionality in that class and only lived their because of its
dependence on the `Bridge` instance, which will soon be removed (see
https://github.com/hypothesis/client/pull/3812). It is now constructed
and used directly by the `Guest` class instead.