Expose supplier_reference to external API

InvenTree/order/api.py

@@ -74,6 +74,7 @@ class POList(generics.ListCreateAPIView):
         data = queryset.values(
             'pk',
             'supplier',
+            'supplier_reference',
             'supplier__name',
             'supplier__image',
             'reference',

InvenTree/order/serializers.py

@@ -19,6 +19,7 @@ class POSerializer(InvenTreeModelSerializer):
         fields = [
             'pk',
             'supplier',
+            'supplier_reference',
             'reference',
             'description',
             'link',