[markverick]

This pull request adds support for email and DNS-01 challenges to the NDNCERT CA server, enabling automated certificate issuance via email and DNS verification. It introduces robust implementations for both challenge types, comprehensive unit tests, and a sample configuration file. Additionally, it ensures that the Docker image includes system CA certificates for secure email delivery.

NDNCERT CA Challenge Support

Email Challenge Implementation:

• Implements ChallengeEmailHandler and SMTPEmailSender in challenge_email.go, supporting secure code generation, email delivery via SMTP (with TLS), and robust validation of user input and code attempts.
• Adds unit tests for the email challenge, covering normal flow, incorrect code attempts, invalid addresses, and SMTP failures in challenge_email_test.go.

DNS-01 Challenge Implementation:

• Implements ChallengeDnsHandler in challenge_dns.go, allowing domain ownership verification via DNS TXT records, with support for test mocking and error handling.
• Adds unit tests for the DNS challenge, including correct and incorrect flows, missing parameters, and DNS lookup failures in challenge_dns_test.go.

Configuration and Integration

• Adds a sample CA configuration file (ca.conf.sample) demonstrating how to configure supported challenges, including SMTP details for email verification.
• Updates the Dockerfile to include system CA certificates, ensuring that outgoing email via SMTP over TLS works correctly in production.

Testing and Policy Validation

• Introduces a test file for CA policy and certificate request validation, ensuring correct CA behavior for certificate storage and request validation in ca_policy_test.go.