PQ/T Hybrid Key Exchange

[tarcieri]

OpenSSH 10.0 introduced support for a hybrid key exchange combining ML-KEM and X25519 called mlkem768x25519-sha256:

https://datatracker.ietf.org/doc/draft-ietf-sshm-mlkem-hybrid-kex/

It would be good to add a crate with support for this key exchange algorithm, leveraging the ml-kem and e.g. curve25519-dalek/x25519-dalek crates