[Python] Support using keyless auth for Azure OpenAI

[pamelafox]

We try to use keyless auth for all our samples, so if I were to use TypeChat in them, it'd need to support keyless auth, aka this constructor:

    openai_client = AsyncAzureOpenAI(
        api_version=api_version,
        azure_endpoint=endpoint,
        azure_ad_token_provider=token_provider,
    )

I'd be happy to just pass in my whole client to typechat as I typically already have it configured correctly for other uses in my app. Trying to match environment variables will be a pain since we have existing environment variable names that arent the same as yours.

[pamelafox]

Keyless auth is now a strict requirement for all our samples, so this a blocker for us.