Make azure_ad_token_provider customizable. (#4931)

## 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.

---------

Co-authored-by: Ryan Sweet <[email protected]>
Co-authored-by: Jack Gerrits <[email protected]>
Co-authored-by: Jack Gerrits <[email protected]>

Author: 3 people

autogen/oai/client.py

@@ -475,8 +475,11 @@ def _configure_azure_openai(self, config: Dict[str, Any], openai_config: Dict[st
         if openai_config.get("azure_ad_token_provider") == "DEFAULT":
             import azure.identity
 
+            azure_ad_token_provider_scope = openai_config.get(
+                "azure_ad_token_provider_scope", "https://cognitiveservices.azure.com/.default"
+            )
             openai_config["azure_ad_token_provider"] = azure.identity.get_bearer_token_provider(
-                azure.identity.DefaultAzureCredential(), "https://cognitiveservices.azure.com/.default"
+                azure.identity.DefaultAzureCredential(), azure_ad_token_provider_scope
             )
 
     def _configure_openai_config_for_bedrock(self, config: Dict[str, Any], openai_config: Dict[str, Any]) -> None: