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/forms.py

@@ -231,7 +231,8 @@ class EditBomItemForm(HelperForm):
             'quantity',
             'reference',
             'overage',
-            'note'
+            'note',
+            'optional',
         ]
 
         # Prevent editing of the part associated with this BomItem