Add Ed25519KeyPair::generate(&rng) function

[vmsp]

Problem:

An Ed25519KeyPair can't be generated without serializing it first, which is a bit awkward. E.g.

use aws_lc_rs::signature::Ed25519KeyPair;

let rng = rand::SystemRandom::new();
let bytes = Ed25519KeyPair::generate_pkcs8v1(&rng).unwrap();
let signing_key_pair = Ed25519KeyPair::from_pkcs8_maybe_unchecked(bytes.as_ref()).unwrap();

Solution:

There should be a Ed25519KeyPair::generate() much like there is a EcdsaKeyPair::generate(). E.g.

use aws_lc_rs::signature::Ed25519KeyPair;

let rng = rand::SystemRandom::new();
let signing_key_pair = Ed25519KeyPair::generate(&rng).unwrap();