Provide a feature to use `ring` instead of `aws-lc-sys`

[Logarithmus]

aws-lc-sys is slow to compile, and most people don't care about FIPS or whatever, so ring is better in most cases.
You need to put default-features = false for rustls, as simple as that.

[f1shl3gs]

This annoying me too, because rustls does not allow users enable ring and aws_lc_rs at the same time. So adding pulsar will panic programe once TLS is used.

See
https://github.com/rustls/rustls/blob/e2f6115548b54e15ec52517208e4b4c6f27aa7db/rustls/src/crypto/mod.rs#L247
https://github.com/rustls/rustls/blob/e2f6115548b54e15ec52517208e4b4c6f27aa7db/rustls/src/crypto/mod.rs#L279