Add CAA record support to GeoDNS

[Copilot]

This PR implements CAA (Certificate Authority Authorization) record support in GeoDNS, allowing domain owners to specify which Certificate Authorities are authorized to issue certificates for their domains.

Changes Made

• Added CAA record type mapping in zones/reader.go
• Implemented CAA record parsing logic with support for flag, tag, value, and weight fields
• Added comprehensive documentation in README.md with examples
• Created test coverage to validate CAA record functionality

CAA Record Format

The implementation supports the standard CAA record format with optional flag and weight fields:

{
  "": {
    "caa": [
      { "tag": "issue", "value": "ca.example.net" },
      { "flag": 0, "tag": "issuewild", "value": "ca.example.net" },
      { "flag": 128, "tag": "iodef", "value": "mailto:[email protected]", "weight": 100 }
    ]
  }
}

Features

• Flag field: Optional, defaults to 0 if not specified
• Tag support: Standard tags like issue, issuewild, and iodef
• Weight support: Full load balancing support like other record types
• Error handling: Comprehensive validation for required fields
• RFC compliance: Follows RFC 6844 specification

Testing

• All existing tests continue to pass
• New comprehensive test case validates CAA record parsing
• Tests cover flag defaults, required field validation, and weight support
• Build successful with no breaking changes

The implementation follows the existing patterns in GeoDNS for record type handling, ensuring consistency with the codebase architecture.

Fixes #102.

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