ML-KEM support (mlkem768x25519-sha256)

OpenSSH now has support for mlkem768x25519-sha256 as a key exchange algorithm, which would be good to add. This is defined in an Internet-Draft which I expect will be stabilized. This algorithm also has the advantage that it will likely be suitable for FIPS mode as well, unlike NTRU.

I've been informed that OpenSSH has code that could be used here and that SUPERCOP may also be a source of reasonable implementations. OpenSSL is adding support for ML-KEM, but it may not be a good idea to wait for that version to be available on all OSes, since people will probably want to use it sooner rather than later (and of course some people will want to use different crypto libraries).