sub schema (SchemaAttributePathInstance) domain model extension + test re-factoring

* incl. new init_internal_schema.sql script (+ generation via code)
I think it's intentional and fine to keep the name "attribute_paths" here and not use "attribute_path_instances" not to overcomplicate the JSON. However, at other places in the Java code, I'd vote for replacing attributePath by attributePathInstance when the type is an instance to avoid confusion. (I'll make an extra comment when I see it next time).