Remove ring dependency with rustls

Right now, our rustls implementation uses ring for its CryptoProvider. Starting with 0.23 (which we now use) rustls can use aws_lc_rs instead, but we have hardwired ring since that's what we had before.

We may want to migrate to using aws_lc_rs by default. At a minimum, we should make it optional.

(I'm assigning this to myself, but others should feel free to steal it if they want.)