Allow using rustls-native-certs instead of webpki-roots

[Shnatsel]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Keep webpki-roots enabled by default, but allow optionally using rustls-native-certs instead.

This is needed for cargo audit which sometimes runs in strange enterprise environments where all HTTPS traffic is intercepted by a MitM proxy, and it's impossible to contact the outside world without using the OS's native HTTPS certificate store.

This is the approach to configuration recommended by @Byron, here: GitoxideLabs/gitoxide#991

I have verified with cargo tree that the features are toggled correctly.

Related Issues

• OpenSSL-less release checklist rustsec/rustsec#939
• When using rustls, allow using rustls-native-tls instead of webpki-roots GitoxideLabs/gitoxide#991

[Shnatsel]

Thanks for the quick review!

I'd appreciate a new release with this change sometime next week.