Allow product customization pricing based on associated line_item configuration

Previously, product customization pricing only had access to the product customization options.  We have a use case where the customization needs access to the overall line_item configuration.

Specifically, we have a customization whose price is a percentage of (the base line_item price plus the price of a separate customization).

This change just associates each product customization with its host line_item before computing customization prices.