jose-jwk: add `JwkParameters` impls

[tarcieri]

The impls of the JwkParameters trait currently live in https://github.com/RustCrypto/elliptic-curves and look like the following:

impl elliptic_curve::JwkParameters for NistP256 {
    const CRV: &'static str = "P-256";
}

These need to be moved into the jose-jwk crate, or the trait needs to be removed in some other way that doesn't require the downstream crates to impl the trait.