feat: Add Azure Virtual Network Gateway Connection construct

[archit017]

Description

This PR adds support for Azure Virtual Network Gateway Connection, which enables connectivity between Virtual Network Gateways and other networking endpoints.

Features

• ✅ Support for three connection types:
  • IPsec (Site-to-Site): Connect VPN Gateway to on-premises networks
  • VNet-to-VNet: Connect two Azure virtual networks via their VPN Gateways
  • ExpressRoute: Connect ExpressRoute Gateway to ExpressRoute circuits
• ✅ Schema-driven validation with discriminated unions for connection types
• ✅ Type-safe configuration with TypeScript
• ✅ Custom IPsec policies support
• ✅ BGP and routing configuration
• ✅ NAT rules support
• ✅ Comprehensive unit tests (913 lines)
• ✅ Integration tests for all connection types
• ✅ Detailed README with usage examples

Changes

• Added VirtualNetworkGatewayConnection construct with full feature support
• Implemented schema validation for all connection types
• Added comprehensive test coverage
• Updated main index exports
• Updated API documentation

Related

This construct complements the existing Virtual Network Gateway construct and enables complete VPN and ExpressRoute connectivity scenarios.

Testing

• Unit tests: All schema validation and property transformation tests passing
• Integration tests: Tests for IPsec, VNet-to-VNet, and ExpressRoute connections included
• Type safety: Full TypeScript type definitions with JSII compliance