[Perform] Introduce new setting for forcing variants to be exported as individual items

Description

Several customers have reported that they want to be able to ignore the products in their catalog and export all variants as separate items instead.

To do this, use the abstractions and structure map interceptor pattern. Partners need to write their own implementation of an interface method whose default implementation is ~500 lines of code with private sub-methods. The source code is provided to the partners for this purpose.