Add a 'uid' field to StockItem model

- To be used for barcode asociation and lookup

InvenTree/stock/models.py

@@ -108,6 +108,7 @@ class StockItem(MPTTModel):
     
     Attributes:
         parent: Link to another StockItem from which this StockItem was created
+        uid: Field containing a unique-id which is mapped to a third-party identifier (e.g. a barcode)
         part: Link to the master abstract part that this StockItem is an instance of
         supplier_part: Link to a specific SupplierPart (optional)
         location: Where this StockItem is located
@@ -295,6 +296,8 @@ class StockItem(MPTTModel):
             }
         )
 
+    uid = models.CharField(blank=True, max_length=128, help_text=("Unique identifier field"))
+
     parent = TreeForeignKey('self',
                             on_delete=models.DO_NOTHING,
                             blank=True, null=True,