fix(engine): avoid requiring stix-related definitions when registering internal entity type

Description

Currently when we define an Entity type using registerDefinition we require setting representative and converter_2_1 functions although it makes no sense for ABSTRACT_INTERNAL_OBJECT types as:

• the Stix converter is not even registered/used
• the representative provider is called only in manager subscribing to the Stream; and no internal objects are ever added to the stream.

On a related note the ModuleDefinition generic type requires a second parameter to be a StixObject (and optionally a third one). Again, it makes no sense for internal objects.

Suggested solution

Either find a way in TS to prevent from passing the representative and converter_2_1 functions when type.category is ABSTRACT_INTERNAL_OBJECT or create a new registerInternalDefinition function.