Update start_new type hints to return str

Author: anthonychu

azure/durable_functions/models/DurableOrchestrationClient.py

@@ -33,7 +33,7 @@ def __init__(self, context: str):
     async def start_new(self,
                         orchestration_function_name: str,
                         instance_id: str,
-                        client_input: object):
+                        client_input: object) -> str:
         """Start a new instance of the specified orchestrator function.
 
         If an orchestration instance with the specified ID already exists, the