[FEATURE] Get an `AsyncOpenAI` client for a serving endpoint.

[mbelak-dtml]

Problem Statement
Currently, WorkspaceClient enables getting an OpenAI client via .serving_endpoints.get_open_ai_client, which is very convenient. However, it does not support AsyncOpenAI, which enables asynchronous calls to serving endpoints.

Proposed Solution

Add a new method (get_async_open_ai_client) with the same functionality as get_open_ai_client , except returning an AsyncOpenAI instance.