### Check for existing issues

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

### The Feature

Allow customizing transport layer (HTTP) by passing a custom `httpx` client as a "client" argument. The customization will provide a simplified path to resolve simple tasks as an alternative to a more complex setup of litellm proxy.

### Motivation, pitch

Today in order to make many atomic manipulations such as: * Refresh bearer token * Inject an observability or analytic action before and after inference call * Dynamically change headers

a developer has to configure and deploy a proxy. While the proxy does this work, it is extra burden which rarely justifies the additional complexity and inherited costs. To be able to do these tasks in-process by implementing a standardized middleware using `httpx` package is a common way to solve these problems.

This FR proposes to add it *only* to `acompletion()` calls to narrow the scope of the change. However, if proved useful, it can be extended to other execution paths as well.

### What part of LiteLLM is this about?

SDK (litellm Python package)

### LiteLLM is hiring a founding backend engineer, ar…