Add means of disabling native-tls

[dureuill]

Hello 👋

Thank you for making the Hugging Face hub, it's super convenient.

This PR adds a means of disabling the native-tls dependency, because we're using rust_tls and really don't want the native one.

It works by adding a new default feature native-tls that adds the existing native-tls optional dependencies (both for ureq and as a direct dependency). So by default this PR doesn't change anything.

However, it is now possible to remove the native-tls dependencies by adding the hf-hub without its default features, and with the online feature enabled.

I checked that the resulting cargo tree doesn't have native-tls at all in this configuration, while native-tls remains when the default features are enabled.

Thank you for reading 🙂

[McPatate]

Closing in favor of #53