[dagster-fivetran] Mark FivetranWorkspace as experimental (#25962)

## Summary & Motivation

As title. All other new APIs for Fivetran are marked as experimental,
but I missed this one.

python_modules/libraries/dagster-fivetran/dagster_fivetran/resources.py

@@ -593,6 +593,7 @@ def get_groups(self) -> Mapping[str, Any]:
         return self._make_request("GET", "groups")
 
 
+@experimental
 class FivetranWorkspace(ConfigurableResource):
     """This class represents a Fivetran workspace and provides utilities
     to interact with Fivetran APIs.