This allows subresources which belong to different API groups than
their parents to be registered in the APIInstaller and REST handlers
installed for them. The specific changes that makes this possible
are:

1. Allow subresource overrides to be specified while registering an
   API group.
2. Use those overrides in the APIInstaller while validating the
   resource/subresource group version to allow subresources which
   belong to a different group to be registered if they have an
   override specified.
3. Use the RESTMapper supplied in the override to map the REST paths
   to the correct subresource storage object, i.e. correct group
   version kinds.