feat(2fa): add PAM and SSH configuration for Google Authenticator

Add configuration files required for two-factor authentication
via Google Authenticator:

- sshd-pam: PAM configuration integrating Google Authenticator
  with standard Unix authentication, using nullok for gradual
  rollout allowing users without 2FA to still authenticate

- sshd-2fa-config: SSH daemon configuration additions enabling
  ChallengeResponseAuthentication and KeyboardInteractive
  authentication methods required for 2FA flow

These configs support the KNEL security baseline requiring 2FA
for SSH access while maintaining backward compatibility during
user onboarding.

Related: KNELServerBuild/ProjectCode/Modules/Security/secharden-2fa.sh

initializers/2fa/configs/sshd-2fa-config

@@ -0,0 +1,11 @@
+# KNEL SSH 2FA Configuration Additions
+# These settings enable two-factor authentication with SSH keys
+
+# Enable challenge-response authentication for 2FA
+ChallengeResponseAuthentication yes
+
+# Enable PAM
+UsePAM yes
+
+# Require both publickey AND keyboard-interactive (2FA)
+AuthenticationMethods publickey,keyboard-interactive