[Feat-Proxy] Slack Alerting - allow using os.environ/ vars for alert to webhook url (#5726)

* allow using os.environ for slack urls

* use env vars for webhook urls

* fix types for get_secret

* fix linting

* fix linting

* fix linting

* linting fixes

* linting fix

* docs alerting slack

* fix get data

litellm/integrations/custom_logger.py

@@ -151,7 +151,13 @@ class CustomLogger:  # https://docs.litellm.ai/docs/observability/custom_callbac
         self,
         data: dict,
         user_api_key_dict: UserAPIKeyAuth,
-        call_type: Literal["completion", "embeddings", "image_generation"],
+        call_type: Literal[
+            "completion",
+            "embeddings",
+            "image_generation",
+            "moderation",
+            "audio_transcription",
+        ],
     ):
         pass