fix: _get returns either str or bytes

Author: heitorlessa

aws_lambda_powertools/utilities/parameters/base.py

@@ -102,7 +102,7 @@ def get(
         return value
 
     @abstractmethod
-    def _get(self, name: str, **sdk_options) -> bytes:
+    def _get(self, name: str, **sdk_options) -> Union[str, bytes]:
         """
         Retrieve parameter value from the underlying parameter store
         """