expose HubApi and model directory configuration for MLX

[tpae]

HubApi provides additional configurations such as download directory, something like this:

    let modelsDirectory = DirectoryPickerService.modelsDirectory()
    let hub = HubApi(downloadBase: modelsDirectory)
    
    return MLXLanguageModel(modelId: modelId, hub: hub)

It should be backwards compatible and shouldn't break existing API.

[mattt]

This looks great. Thank you for contributing this, @tpae!

FYI — I'm working to replace HubApi in swift-transformers with swift-huggingface, which should provide better performance and ergonomics for managing models. When the time comes, we can update this code to that new API.