Allow users to pass their own custom rustls::crypto::CryptoProvider

[oleonardolima]

As mentioned by Lloyd's in #135 the ideal solution would be to allow users to pass their own custom CryptoProvider.

We decided not to make this change part of the PR, but handle it as a standalone issue/PR.

I started addressing it in #138

Can we just expose the rustls provider API here: https://docs.rs/rustls/latest/rustls/crypto/struct.CryptoProvider.html so I can pass in my own provider?
We can have a rustls-aws-lc feature that adds a constructor that uses that. Can do the same for rustls-ring. But I'd like to able to fix this kind of issue in the future without having to wait for a merge.

Originally posted by @LLFourn in #135 (comment)

[oleonardolima]

Oh, I didn't notice we already had #137, closing this one in favor of #137.