Switch default cryptography provider to aws-lc-rs

[cpu]

The current default is ring: https://github.com/rustls/webpki/blob/dba3b84bcf810612a83c8fd3c750d3c4106e7b9e/Cargo.toml#L75

This is incongruous with rustls as of 0.24 and since this crate is almost entirely designed to be used with rustls we should keep the two in sync. While easily fixed, it helps avoid situations like https://github.com/rustls/rustls-platform-verifier/issues/102

I propose we harmonize the default when the next occasion for a major release arrives.

[djc]

I think we should probably avoid having a crypto implementation in the default features in the next breaking release (though it might not matter much for this crate).