Fix #3372 by adding digital goods rounding item

[luzat]

Rounding differences were previously added as physical goods, which changed the properties of the cart. Now, if the cart does not contain physical items, the extra line item will be added as a digital goods item.

Issue: #3372

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Description

Rounding differences can change the basket to contain physical goods and lead to additional errors such as order failure. This PR add the extra line item for rounding errors as a digital goods item by default, except if there are already some physical goods in the basket.

It may be viable to always add the rounding difference as a digital goods item, but I decided to change as little as possible compared to the current handling.

Steps to Test

1. Create an order with only digital goods which introduces rounding differences (I guess because PayPal is receiving net values; here, this happens with a VAT of 19% and two items of 9.50 gross value each).
2. An extra_line item Subtotal mismatch will be added by default by PurchaseUnitSanitizer. This item is now a digital product.
3. Redo step 1 with at least some physical goods.
4. A physical goods extra_line item Subtotal mismatch will be added by default by PurchaseUnitSanitizer.

Changelog Entry

Fix - Rounding items are digital goods unless the basket contains physical goods #3372