Add comprehensive SSL certificate methods to cert_manager.sh

Extended cert_manager.sh with multiple certificate issuance methods:

CERTBOT Support:
- Standalone HTTP validation
- Webroot validation
- DNS validation for Cloudflare, AWS Route53, Google Cloud DNS, DigitalOcean
- Automatic installation of Certbot and DNS plugins
- Auto-renewal via systemd timer

ACME.SH Extensions:
- AWS Route53 DNS validation
- Google Cloud DNS validation
- DigitalOcean DNS validation
- ZeroSSL CA support as alternative to Let's Encrypt

Additional Features:
- Self-signed certificate generation for testing/development
- Updated interactive menu with organized sections
- Comprehensive CLI with new commands:
  * acme-http, acme-cloudflare, acme-route53, acme-gcloud, acme-digitalocean, acme-zerossl
  * certbot-standalone, certbot-webroot, certbot-cf, certbot-route53, certbot-gcloud, certbot-do
  * selfsigned
- Backwards compatibility maintained for existing commands
- Improved help text and usage documentation

All methods follow the same pattern:
- Automatic dependency installation
- User-friendly prompts
- Certificates stored in /root/cert/{domain}
- Automatic renewal setup
- Detailed logging and error handling

Author: claude