Removed ssh password auth

[okankoAMZ]

Description of the issue

This PR addresses the need to disable SSH password authentication, challenge-response authentication, and keyboard-interactive authentication on EC2 instances for enhanced security.

Description of changes

This change adds a user_data script to the AWS EC2 instance resource in the Terraform configuration. The script performs the following actions:

1. Disables password authentication for SSH
2. Disables challenge-response authentication for SSH
3. Disables keyboard-interactive authentication for SSH
4. Restarts the SSH service to apply the changes

Note: ChallengeResponseAuthentication was replaced by KbdInteractiveAuthentication in newer versions of SSH.
Source: https://askubuntu.com/questions/1403846/ssh-challengeresponseauthentication-in-22-04-ubuntu

License

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Tests

• Automated test run: https://github.com/aws/amazon-cloudwatch-agent/actions/runs/12302119452/job/34390526816
• Manual verification on the host:

[root~]# cat /etc/ssh/sshd_config | grep Password
# Explicitly disable PasswordAuthentication. By presetting it, we
PasswordAuthentication no
PermitEmptyPasswords no
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication, then enable this but set PasswordAuthentication

[the-mann]

looks good overall. I wonder if we should add some logging as well so debugging is easier if something happens.