feat: add --validate flag for configuration validation

[bdmorin]

Summary

This PR adds command-line configuration validation functionality to Gatus, addressing a common need for validating configurations in CI/CD pipelines and deployment scripts without requiring full application startup.

Changes

• Added --validate flag: Validates configuration file and exits with appropriate status code
• Added --config flag: Allows specifying configuration file path (overrides environment variables)
• Detailed validation output: Shows endpoint count, alerting providers, storage type, and external endpoints
• Proper exit codes: Returns 0 for valid config, 1 for invalid config (CI/CD ready)
• Backward compatibility: Maintains existing environment variable support

Usage

# Validate configuration file
./gatus --validate --config /path/to/config.yaml

# Use with environment variable
GATUS_CONFIG_PATH=/path/to/config.yaml ./gatus --validate

# CI/CD integration
if ! ./gatus --validate; then
    echo "Configuration validation failed"
    exit 1
fi

Example Output

Validating configuration: config.yaml
Configuration validation passed!
  - Endpoints: 7
  - Storage: memory
Configuration is valid and ready to use

Benefits

• CI/CD Integration: Enable configuration validation in deployment pipelines
• DevOps Friendly: Quick validation without full application startup
• Error Prevention: Catch configuration issues before deployment
• Professional Workflow: Supports modern infrastructure-as-code practices

Testing

• ✅ Validates configuration using existing config.LoadConfiguration() logic
• ✅ Proper error handling and exit codes
• ✅ Help flag displays new options correctly
• ✅ Backward compatibility maintained
• ✅ No breaking changes to existing functionality

This addresses a gap that many monitoring tools have - the ability to validate configuration without starting the full service, which is essential for automated deployment workflows.