Add possibility to set custom models

[PierreMesure]

We just got really good Swedish Whisper finetunes last week and the model maintainers decided to use a single repo with all the variants of a model (transformers, ctranslate2, ggml, onnx).

Unfortunately, HuggingFace model cards don't seem to support having several library_name so they set it to transformers and not ctranslate2.

• Would it be possible to add an env variable where extra models can be specified that are added to the list of available models?
• Would it be possible to allow for API calls specifying any model from HuggingFace, regardless if they have the right library_name? (I haven't tested so this might already be possible)