Support for fine-tuned OpenAI models

It's currently non-trivial to add a new model to the "Model Version" dropdown in the model settings.

Adding a new value to the enum property in ChatGPTSettings, followed by adding a new entry to the NativeLLM enum and then rebuilding the frontend, is not sufficient. I'd like to add a new `gpt-3.5-turbo-instruct` model and a custom fine-tuned one.

What are the steps?