Add external_id to CatalogSaleSerializer

- Include external_id field in CatalogSaleSerializer response
- Allows API clients to see if catalog sale has been synced to Tryton
- Maintains consistency with SaleSerializer which already includes external_id
- Backward compatible change (field is nullable)

tienda_ilusion/don_confiao/serializers/sales.py

@@ -58,6 +58,7 @@ class CatalogSaleSerializer(serializers.ModelSerializer):
             "date",
             "catalogsaleline_set",
             "total",
+            "external_id",
         ]
 
     def create(self, validated_data):