PayPal item list should include product SKU

[techjp]

There should be an option in the module config to include the product SKU when sending item lists to PayPal. It should also be configurable exactly what is sent in this column.

Some merchants may wish to send the value from the product table's reference column, ean13 column, upc column, or supplier_reference column. Having a SKU in the PayPal data makes it easier to cross-reference PayPal payments with TB order & payment data.

To include the reference value this line of code needs to be added to the PayPalRestApi.php file in several places around line 350 where the $aItems array object is built:

'sku'      => $cartItem['reference'],