add unit_price_price_and_unit to line_item serializer

*  unit_price_price_and_unit refers UnitPrice which refers to other attributes
that belong to variant and product.
Instead of collecting all the attributes that were using indirectly by the method and adding them to the serializers
I perefered to serialize the result of the method call.

app/models/invoice/data_presenter/line_item.rb

@@ -4,7 +4,7 @@ class Invoice
   class DataPresenter
     class LineItem < Invoice::DataPresenter::Base
       attributes :added_tax, :currency, :included_tax, :price_with_adjustments, :quantity,
-                 :variant_id
+                 :variant_id, :unit_price_price_and_unit
       attributes_with_presenter :variant
       array_attribute :tax_rates, class_name: 'TaxRate'
       invoice_generation_attributes :added_tax, :included_tax, :price_with_adjustments,

app/serializers/invoice/line_item_serializer.rb

@@ -3,7 +3,7 @@
 class Invoice
   class LineItemSerializer < ActiveModel::Serializer
     attributes :id, :added_tax, :currency, :included_tax, :price_with_adjustments, :quantity,
-               :variant_id
+               :variant_id, :unit_price_price_and_unit
     has_one :variant, serializer: Invoice::VariantSerializer
     has_many :tax_rates, serializer: Invoice::TaxRateSerializer
   end