feat(proxy_server.py): add support for blocked user lists (enterprise-only)

[krrishdholakia]

Addresses - #2136

Enable Blocked User Lists

If any call is made to proxy with this user id, it'll be rejected - use this if you want to let users opt-out of ai features

litellm_settings: 
     callbacks: ["blocked_user_check"] 
     blocked_user_id_list: ["user_id_1", "user_id_2", ...]  # can also be a .txt filepath e.g. `/relative/path/blocked_list.txt` 

How to test

curl --location 'http://0.0.0.0:8000/chat/completions' \
--header 'Content-Type: application/json' \
--data ' {
      "model": "gpt-3.5-turbo",
      "messages": [
        {
          "role": "user",
          "content": "what llm are you"
        }
      ],
      "user_id": "user_id_1" # this is also an openai supported param 
    }
'

[railway-app]

This PR was not deployed automatically as @krrishdholakia does not have access to the Railway project.

In order to get automatic PR deploys, please add @krrishdholakia to your team on Railway.

[railway-app]

This PR was not deployed automatically as @krrishdholakia does not have access to the Railway project.

In order to get automatic PR deploys, please add @krrishdholakia to your team on Railway.

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