Add the tax_category_id for each adjustement

- This tax_category_id could be null
 - Special case if the originator is a Spree::TaxRate: then use the tax_category_id of the originator

Co-Authored-By: Matt-Yorkley <9029026+Matt-Yorkley@users.noreply.github.com>

app/serializers/api/adjustment_serializer.rb

@@ -4,6 +4,15 @@ module Api
   class AdjustmentSerializer < ActiveModel::Serializer
     attributes :id, :amount, :label, :eligible,
                :adjustable_type, :adjustable_id,
-               :originator_type, :originator_id
+               :originator_type, :originator_id,
+               :tax_category_id
+
+    def tax_category_id
+      if object.originator_type == "Spree::TaxRate"
+        object.originator.tax_category_id
+      else
+        object.tax_category_id
+      end
+    end
   end
 end