feat(proxy_server.py): allow admin to return rejected response as string to user

Closes https://github.com/BerriAI/litellm/issues/3671

litellm/integrations/custom_logger.py

@@ -4,7 +4,7 @@ import dotenv, os
 
 from litellm.proxy._types import UserAPIKeyAuth
 from litellm.caching import DualCache
-
+from litellm.utils import ModelResponse
 from typing import Literal, Union, Optional
 import traceback
 
@@ -64,8 +64,17 @@ class CustomLogger:  # https://docs.litellm.ai/docs/observability/custom_callbac
         user_api_key_dict: UserAPIKeyAuth,
         cache: DualCache,
         data: dict,
-        call_type: Literal["completion", "embeddings", "image_generation"],
-    ):
+        call_type: Literal[
+            "completion",
+            "text_completion",
+            "embeddings",
+            "image_generation",
+            "moderation",
+            "audio_transcription",
+        ],
+    ) -> Optional[
+        Union[Exception, str, dict]
+    ]:  # raise exception if invalid, return a str for the user to receive - if rejected, or return a modified dictionary for passing into litellm
         pass
 
     async def async_post_call_failure_hook(