Add support for hybrid key exchange methods

[scott-xu]

Suggest adding support for sntrup761x25519-sha512 and mlkem768x25519-sha256 key exchange method which is described at https://www.ietf.org/archive/id/draft-ietf-sshm-ntruprime-ssh-01.html and https://www.ietf.org/archive/id/draft-kampanakis-curdle-ssh-pq-ke-05.html

I created 2 PRs in SSH.NET. See sshnet/SSH.NET#1562 and sshnet/SSH.NET#1563

@tmds Let me know if you want me port the two PRs to this repository.

[tmds]

@scott-xu it would be very interesting to have support for this, and thank you for offering to implement it!

I'm not sure if the abstractions we have are fit to plug this in. We'll figure that out as we go.
Let's start with sntrup761x25519-sha512, and tackle mlkem768x25519-sha256 after that.

For tests, please add something like https://github.com/tmds/Tmds.Ssh/blob/main/test/Tmds.Ssh.Tests/HostKeyAlgorithmTests.cs but for setting KeyExchangeAlgorithms instead of ServerHostKeyAlgorithms.

I'm finishing off some things to make a December release, and this can be part of the January release.

[scott-xu]

Let me try curve25519-sha256 first.