Make azure_ad_token_provider customizable.

[chinganc]

Why are these changes needed?

Currently, when using azure openai, azure_ad_token_provider in OAI_CONFIG_LIST can only take a value of "DEFAULT" which uses the default token provider "https://cognitiveservices.azure.com/.default".

This patch would allow users providing custom token providers by setting azure_ad_token_provider to other values.

Checks

• [ x] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [ x] I've made sure all auto checks have passed.

[ekzhu]

You can run pre-commit --all-files to fix the formating