ruby : add Core ML support

[KitaitiMakoto]

Currently, on Ruby bindings, model name (like "base") can be used to load model file instead of URI and is downloaded automatically:

whisper = Whisper::Context.new("base")
# Automatically downloads model file from https://huggingface.co/ggerganov/whisper.cpp/resolve/main/ggml-base.bin

But you need generate or download correspondent Core ML model files and move them to the directory the same to normal models manually.

This pull request adds auto-download feature for Core ML models.

Thank you.