Make download models script a subcommand for the binary

[dit7ya]

Hi,

Thanks for this awesome project. I am wondering if you could make the bash script part of the binary itself as a subcommand? In that way it will be self-contained and much easier to package for various package-managers.

[ggerganov]

We could probably add a whisper_model_download(const char * name, const char * path) function to the C-style API.
It can try to download the selected model into the specified path after checking that it is not already there. We probably have to use std::system to try to call curl or wget if available. Not sure what to do if they are not available. Any suggestions? I don't want to add a third-party dependency to the library, just to be able to download the model files.

[dit7ya]

Thanks. I think it is fine to depend on curl or wget, and show a failed message if they are not available.