CHEF-8598: Add support for curve25519 key exchange

[ahasunos]

Description

This pull request introduces support for the curve25519 key exchange to address an error associated with key exchange algorithm negotiation.

Here is the issue encountered when attempting to SSH into a machine (an Ubuntu EC2 instance in my case) which is configured to exclusively support curve25519 for key exchange.

Transport error, can't connect to 'ssh' backend: SSH command failed (could not settle on kex algorithm
Server kex preferences: curve25519-sha256@libssh.org,kex-strict-s-v00@openssh.com
Client kex preferences: ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1)

Related Issue

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New content (non-breaking change)
• [ ] Breaking change (a content change which would break existing functionality or processes)

Checklist:

• [x] I have read the CONTRIBUTING document.

[Vasu1105]

If anyone has doubt why this is in test group https://github.com/inspec/train/pull/439