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.
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