Feature: #277 - Add TaxClass Interface

See Issue #277


Parsing the tax classes from TypoScript is moved to an own class implementing of the \Extcode\Cart\Service\TaxClassServiceInterface. If the tax classes for a cart comes from an API the Interface can be used to load the tax classes through .

More information about the \Extcode\Cart\Service\TaxClassServiceInterface can be found in the tax_class_service_interface documentation.