Add support for KEX `sntrup761x25519-sha512` alongside existing `mlkem768x25519-sha256`

[Blacks-Army]

Currently, russh supports the post-quantum hybrid KEX mlkem768x25519-sha256, which is great.

It would be very useful to also support sntrup761x25519-sha512 (and optionally sntrup761x25519-sha512@openssh.com) so russh clients and servers can connect to OpenSSH servers offering this PQ KEX.