Change bool to BOOL index name when accessing boolean value in dynamodb. (#176)

It seems that when accessing boolean value from dynamodb index name `bool` is used instead of correct one `BOOL`.
Have a look at https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_AttributeValue.html to see list of available attribute values

Author: whisller

Diff for: aws_lambda_powertools/utilities/data_classes/dynamo_db_stream_event.py

@@ -35,7 +35,7 @@ def bool_value(self) -> Optional[bool]:
         Example:
             >>> {"BOOL": True}
         """
-        item = self.get("bool")
+        item = self.get("BOOL")
         return None if item is None else bool(item)
 
     @property