support building without ring or aws-lc-rs

[FlorianUekermann]

Since aws-lc-rs support was added, most of the library is built around rustls::crypto::CryptoProvider to be generic over aws-lc-rs and ring. It would be nice to use something similar for the remaining bits, which covers more of the ring functionality than rustls::crypto::CryptoProvider. Maybe something like this already exists in the ecosystem... ...or maybe we should implement our own provider.
The crypto provider choice also trickles down to rcgen, which may be a problem.

@joshtriplett : Do you have any input on this?

[joshtriplett]

@FlorianUekermann You're talking about having enough of an abstraction that you could use the crypto provider for all your crypto needs, and thus support any future crypto provider that rustls does (e.g. https://github.com/RustCrypto/rustls-rustcrypto when ready)?

That seems potentially reasonable. You might put together a list of what you're using from ring / aws-lc-rs that rustls doesn't provide, and ask upstream about having that functionality available without going directly to ring / aws-lc-rs.