Marker extension identifying a module as a valid characteristic resource.
Also synthesises default :create and :update actions on
BaseCharacteristic-derived resources from the resource's declared public
attributes — :create accepts [:name | <public_attrs>] with
:instance_id / :feature_id arguments and corresponding
manage_relationship changes; :update accepts <public_attrs> only.
Consumers may declare their own :create / :update actions to override
the synthesised ones.