Describe the pull request

Adds a new configuration option [server] SSH_SERVER_ALGORITHMS for specifying the list of accepted key exchange algorithms for connections to builtin SSH server. Defaults to rsa, ecdsa, ed25519.

Link to the issue: fixes #6638

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code.

Test plan

1. Start built-in SSH server at port 2222

   2023/02/12 12:38:04 [ INFO] SSH server started on 0.0.0.0:2222
   2023/02/12 12:38:04 [TRACE] SSH server cipher list: [aes128-ctr aes192-ctr aes256-ctr aes128-gcm@openssh.com arcfour256 arcfour128]
   2023/02/12 12:38:04 [TRACE] SSH server MAC list: [hmac-sha2-256-etm@openssh.com hmac-sha2-256 hmac-sha1]
   2023/02/12 12:38:04 [TRACE] SSH server algorithms: [rsa ecdsa ed25519]
   

2. Do a host key scan:

   → ssh-keyscan -p 2222 localhost
   # localhost:2222 SSH-2.0-Go
   [localhost]:2222 ssh-rsa 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
   # localhost:2222 SSH-2.0-Go
   [localhost]:2222 ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBGSideZF+SUQk0PmgSAxgOXw5MDnkYdybso1niF6DFtPlqRakBnLZ8tA6LPuGYUnIY2KsZLIXjf6Ky+OX31aQhg=
   # localhost:2222 SSH-2.0-Go
   [localhost]:2222 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICR0iqGkPT1p75RQUaDXse1oW6zIqLuFUYY1pqjQnlVK
   # localhost:2222 SSH-2.0-Go
   # localhost:2222 SSH-2.0-Go