update storage api

sdk/agentserver/azure-ai-agentserver-agentframework/azure/ai/agentserver/agentframework/models/agent_thread_repository.py

@@ -33,11 +33,13 @@ async def set(self, conversation_id: str, thread: AgentThread) -> None:
         """
 
 
-class AgentThreadStorage:
-    async def save(self, conversation_id: str, serialized_thread: str) -> None:
+class AgentThreadStorage(ABC):
+    @abstractmethod
+    async def save(self, conversation_id: str, serialized_thread: Any) -> None:
         pass
 
-    async def get(self, conversation_id: str) -> Optional[str]:
+    @abstractmethod
+    async def get(self, conversation_id: str) -> Optional[Any]:
         pass