idea: Add TogetherAI as a model provider

[sxflynn]

Problem Statement

As a TogetherAI user, I'd like to be able to make calls to TogetherAI and the models they offer.

Feature Idea

TogetherAI provides an OpenAI API compatible chat completion endpoint with a pay per token system and has a bunch of open models like Llama etc, not to dissimilar from the likes of Anyscale.