Provide a getter method for adjusted price of order items

Problem/motivation

While working with adjustments, and specifically tax adjustments, I realize we need the ability to get an order item's adjusted price, not just the total (unit price * qty). For example, if there are order item adjustments, the unit price will be recorded with qty as the order item total, but the adjusted price will take those order item adjustments into account. Tax calculation, in this case, is concerned with the adjusted price. The order will apply those adjustments in their own adjustment item on the order, but the tax service needs the adjusted price.

Proposed resolution

Add getTotalAdjustedPrice to OrderItemInterface
Implement getTotalAdjustedPrice as order item total price + adjustments