Add externalized inverse and index supplies

Externalized supplies will be needed if we want to support use cases where a user doesn't have inverse and index shadow variables in the domain model.

This can happen if one or both shadow variables are not needed for in the constraint provider.
 

• Try various combinations (index is domain-internal, inverse is externalized).
• Index depends on inverse, don't forget about this.
• Add new Testdata domain.
• Maybe try to modify listvariable task assigning domain to test it (if it's possible to rewrite constraint provider to work without inverse variable).