Feature: #26 - Add addToCart form framework finisher

See forge#26

Description

In order to allow to individualize events when adding them to the cart, a new addToCart finisher for the form framework allow to load a form and submit the form with the selected event. The fields are handled as frontend variants in the cart product. They have no intended impact on the price or stock handling.

An example form template 'Cart Events - Example' can be used to create different forms for different events. It can also serve as a template for manually creating forms.