Dynamic Price based on Variant for Line Item factories

[kennyadsl]

At the moment, our line item factory has a static value as the default for its price, which is BigDecimal('10.00').

This has some advantages, like give for granted the price and avoid making lot of resources traversing for complex cases but it also has cons, like forcing us to stub the line item price, when we need it to be something different from the default, as happening here.

We should understand if it's possible to make this price dynamic, based on the price of the variant associated with that line item. We should also deprecate the current line item factory, in order to allow backward compatibility for users that are relying on our core factories.