Support custom fine-tuned models

[mme]

Add support for custom fine-tuned models.

See: https://platform.openai.com/docs/guides/fine-tuning/use-a-fine-tuned-model

This should already work, but needs improvements in Typescript (gives a compiler error now).

We could simply add a CustomModel class that wraps the ID of the custom model and accept the type OpenAIModel | CustomModel