Add checkdomain.de DDNS provider support

[Copilot]

This PR adds support for checkdomain.de as a DDNS provider, implementing their REST API integration as mentioned in the issue comments.

Changes Made

• New Provider Implementation: Created pkg/ddns/providers/checkdomain.go with full REST API support
• Provider Registration: Added Checkdomain to the updaters registry in update.go
• Documentation Update: Updated README.md to include Checkdomain in the supported providers list
• Build Cleanup: Updated .gitignore to prevent committing build artifacts and temporary files

Implementation Details

The checkdomain.de provider follows the established patterns used by other REST API-based providers in the codebase:

• Uses Bearer token authentication for API access
• Supports both creating new DNS records and updating existing ones
• Implements proper error handling with HTTP status code validation
• Follows the REST API structure documented at developer.checkdomain.de
• Uses the standard API endpoint pattern with proper JSON request/response handling

API Structure

The provider accepts the following configuration parameters:

• APIToken: Bearer token for authentication
• Domain: The domain to update
• RecordName: The DNS record name (subdomain)
• RecordType: DNS record type (defaults to "A")
• TTL: Time-to-live value (defaults to 3600 seconds)

Testing

• Build passes successfully with no compilation errors
• Provider is properly registered and available in the provider list (verified programmatically)
• Code passes go vet and go fmt checks
• Total provider count increased from 37 to 38

The implementation is ready for use and can be extended with additional API features as needed based on the full checkdomain.de API documentation.

Fixes #15.

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