Add flexible VNet and subnet deployment support

- Support three scenarios: new VNet, existing VNet with new subnets, existing VNet with existing subnets
- Add parameters: useExistingVnet, createSubnets, vnetAddressPrefix, subnet addresses
- Add update_existing_subnets nested deployment for subnet delegation updates
- Remove hardcoded resource group/subscription IDs
- Remove CDN-related resources

Author: Keshava Kandikonda