refactor: Update volatility parameter type in udwf method signature to support Volatility enum

Author: kosiew

python/datafusion/udf.py

@@ -632,7 +632,7 @@ def __call__(self, *args: Expr) -> Expr:
     def udwf(
         input_type: pa.DataType | list[pa.DataType],
         return_type: pa.DataType,
-        volatility: str,
+        volatility: Volatility | str,
         name: Optional[str] = None,
     ) -> Callable[..., WindowUDF]: ...
 
@@ -642,7 +642,7 @@ def udwf(
         func: Callable[[], WindowEvaluator],
         input_type: pa.DataType | list[pa.DataType],
         return_type: pa.DataType,
-        volatility: str,
+        volatility: Volatility | str,
         name: Optional[str] = None,
     ) -> WindowUDF: ...