Add X25519MLKEM768 hybrid key agreement option

[jschneider-bensch]

This PR addresses the addition of an ML-KEM based hybrid key agreement described in issue #124 .

More specifically, it uses the libcrux implementation of X25519MLKEM768 to offer one new ciphersuite

/// `TLS_CHACHA20_POLY1305_SHA256`
/// with
/// * X25519MlKem768 for key exchange (cf. https://datatracker.ietf.org/doc/draft-kwiatkowski-tls-ecdhe-mlkem/)
/// * EcDSA P256 SHA256 for signatures
pub const SHA256_Chacha20Poly1305_EcdsaSecp256r1Sha256_X25519MlKem768: Algorithms = Algorithms::new(
    HashAlgorithm::SHA256,
    AeadAlgorithm::Chacha20Poly1305,
    SignatureScheme::EcdsaSecp256r1Sha256,
    KemScheme::X25519MlKem768,
    false,
    false,
);

It does not implement the SecP256r1MLKEM768 ciphersuite described by the Internet Draft.

Fixes #124

[franziskuskiefer]

Thanks, lgtm when the name is fixed.

[franziskuskiefer]

I manually triggered hax, which was auto-disabled. https://github.com/cryspen/bertie/actions/runs/12025538546/job/33522857963
Merging this without re-triggering everything.