Adding Telnyx API for open source model support

[jgordley]

This PR adds support for the Telnyx API, an OpenAI compatible API that serves a variety of open-source HuggingFace models like meta-llama/Meta-Llama-3-70B and NousResearch/Nous-Hermes-2-Mixtral-8x7B-DPO.

Not a whole lot of changes needed to be made, namely just the addition of the TELNYX_API_KEY requirement for using that provider and the telnyx_base_url and telnyx_base_model config attributes.

Please let me know if you are open to this addition, as it opens up the possibility of testing out README generation performance with the latest open-source models. Thanks!

[eli64s]

@jgordley Hey Jack, my bad for the delay. Will review your PR sometime this week and get back to you. Thank you for the addition!