### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

Passthrough endpoint works relatively transparent and allows users to do the customization on their part but it is only availabe for Pre-api hook. If a user have to expose an non openai endpoint lets say "/tokenize" and have a specific format for request & response, inorder to make the translation of request & reponse, hooks would be used. The pre-api hook is really helpful to dynamically make the translation of the request based on the llm provider but due to the absence of post-api hook, the response aren't translated to specific fixed format.

The proposal is to add the post-api hook for the passthrough endpoint.

**Any challenges:** 1. Post api hook doesn't have any call_type argument, which means we are at our own in filtering the responses and this hook would get triggered for all the passthrough endpoint configuration. 2. What kind of dict do we need to provide in data argument? It has to be either request_data or generated kwargs. kwargs has more information which allows users to do better customization on the custom callbacks. 3. If t…