Add Caddy import support

[Copilot]

This PR adds support for importing SSL certificates generated by Caddy Server into UniFi OS Server, addressing issue requirements to use existing Caddy-generated certificates without needing to generate new ones.

Changes Made

Script Updates

• Added caddy as a supported certificate provider alongside existing certbot and acme options
• Implemented Caddy certificate directory resolution for the standard path: /var/lib/caddy/.local/share/caddy/certificates/acme-v02.api.letsencrypt.org-directory/{domain}/
• Added support for Caddy's certificate file naming convention ({domain}.crt and {domain}.key)
• Updated validation logic to check for Caddy data directory existence
• Enhanced certificate installation logic to handle Caddy's full-chain certificate format
• Updated help text and error messages to include Caddy option

Documentation Updates

• Added comprehensive Caddy support section to README.md with prerequisites and configuration examples
• Updated all command-line examples to demonstrate Caddy usage
• Added Caddy-specific automation examples for crontab setup
• Updated feature list and provider options throughout documentation

Usage

Basic Usage

# Import existing Caddy certificates
./unifi-osserver-ssl-import.sh --provider=caddy

# With verbose output for troubleshooting
./unifi-osserver-ssl-import.sh --provider=caddy --verbose

Automation

# Auto-import Caddy certificates twice daily
5 */12 * * * root /usr/local/bin/unifi-osserver-ssl-import.sh --provider=caddy >> /home/import_log.txt 2>&1

Prerequisites

• Caddy server must be installed and configured
• Caddy must have already generated certificates for your domain
• Certificates should exist at the expected Caddy path structure

Testing

The implementation has been thoroughly tested with:

• Mock certificate directory structure validation
• Error handling for missing certificate directories
• Shell script linting (shellcheck)
• Command-line argument parsing verification

This change maintains full backward compatibility with existing certbot and acme providers while adding seamless Caddy integration.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add Caddy import support</issue_title>
<issue_description>We have an SSL certificate generated by https://caddyserver.com/ located at /var/lib/caddy/.local/share/caddy/certificates/acme-v02.api.letsencrypt.org-directory/unifios.synergi.tech/

We need to add support for this to be imported into unifi os. Modify the existing .sh file to add Caddy support. We should not need to generate new SSL certificates; we only need to import the new ones, as Caddy handles generation.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #1

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[willpower232]

I'd probably reduce the automation to 5 */12 * * * root /usr/local/bin/unifi-osserver-ssl-import.sh --provider=caddy so there isn't one giant log file but sure