Allow passing in openai/anthropic/etc clients

[alecf]

There are a number of 3rd party services that record + eval your prompts, and they usually act as drop-in replacements for the various OpenAI/Anthropic/etc clients. It would be nice if token.js allowed you to pass in the client to replace the default import

i.e.

import { OpenAI } from '@libretto/openai'

const librettoOpenAI = new OpenAI({...});
const tokenjs = new TokenJS({ openai: librettoOpenAI });