Add "optional" field to BomItem

- Defaults to False
- Indicates that the BomItem is "optional" for a build
- Will be used in the future when calculating if a Build output is fully allocated!

InvenTree/part/serializers.py

@@ -403,6 +403,7 @@ class BomItemSerializer(InvenTreeModelSerializer):
             'quantity',
             'reference',
             'price_range',
+            'optional',
             'overage',
             'note',
             'validated',