Enhance plugin SN validation (#7942)

* Update function signature for 'validate_serial_number'

- Pass through stock item parameter
- Required if we want to exclude a particular item from that test

* Update documentation

* Docs fixes

* Add type annotations

src/backend/InvenTree/plugin/samples/integration/validation_sample.py

@@ -121,7 +121,7 @@ class SampleValidatorPlugin(SettingsMixin, ValidationMixin, InvenTreePlugin):
             if d >= 100:
                 self.raise_error('Value must be less than 100')
 
-    def validate_serial_number(self, serial: str, part):
+    def validate_serial_number(self, serial: str, part, stock_item=None):
         """Validate serial number for a given StockItem.
 
         These examples are silly, but serve to demonstrate how the feature could be used