Python: Create/Get Agent API for OpenAI Assistants (#3208)

* Added provider implementation

* Added example with response format

* Small improvements

Author: dmytrostruk

python/packages/core/agent_framework/openai/__init__.py

@@ -1,6 +1,7 @@
 # Copyright (c) Microsoft. All rights reserved.
 
 
+from ._assistant_provider import *  # noqa: F403
 from ._assistants_client import *  # noqa: F403
 from ._chat_client import *  # noqa: F403
 from ._exceptions import *  # noqa: F403