Allow line_items update using external_price

[gciotola]

Closes #

What I did

When we have a line_item we don't know if has been created with an external price, since _external_price is a trigger attribute and nothing is saved at the item level.

What happens now is that when we update a quantity with the <LineItemQuantity> component the PATCH request updates the price using the regular price list.

With this PR we are exposing a hasExternalPrice prop to <LineItemQuantity> so that the consumer app can instruct it to patch the line_item using _external_price: true.

How to test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests).
• Make sure to add/update documentation regarding your changes.
• You are NOT deprecating/removing a feature.

[acasazza]

Well done!