[Client] Add Auth headers to the python async routes

[seanshi-scale]

Pull Request Summary

Add Basic Auth headers when people use the client's python async routes

Test Plan and Usage Guide

Setting LLM_ENGINE_BASE_PATH to a self-hosted LLM Engine:

import asyncio
from llmengine import Completion
asyncio.run(Completion.acreate(model='llama-2-7b-chat', prompt='prompt'))

works. Without the change, we get

<stack trace>
llmengine.errors.UnauthorizedError: No authentication was passed in