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Add support for existing PostgreSQL databases with cross-VNet connectivity

This PR extends the terraform-azure-datafold module to support using existing PostgreSQL databases located in different VNets and resource groups, while maintaining backward compatibility with the existing database creation functionality.

✨ New Features

• Existing Database Support: New use_existing_database flag allows referencing existing PostgreSQL Flexible Servers instead of creating new ones
• Cross-VNet Connectivity: Automatic VNet peering setup when existing database is in a different VNet
• Private Endpoint Integration: Creates private endpoints for secure connectivity to existing databases
• DNS Integration: Leverages existing private DNS zones for proper name resolution
• Resource Name Overrides: Added override variables for new networking resources (private endpoints, VNet peering)

🔧 Configuration

module "azure" {
  # ... existing config ...
  
  # Enable existing database usage
  use_existing_database = true
  
  # Database details
  existing_database_resource_group_name = "shared-services-rg"
  existing_postgresql_server_name       = "prod-postgres-01" 
  existing_postgresql_database_name     = "datafold"
  
  # VNet peering configuration
  existing_vnet_resource_group_name = "shared-services-network-rg"
  existing_vnet_name                = "shared-services-vnet"
  existing_database_subnet_name     = "database-subnet"
}

🛠️ Technical Changes

• Added new variables and validation for existing database configuration
• Implemented conditional resource creation using count based on use_existing_database flag
• Added data sources for existing PostgreSQL server, VNet, and private DNS zone
• Created bidirectional VNet peering resources
• Updated outputs to handle both created and existing database scenarios
• Fixed missing resource_group_name_override variable and invalid data source issues

📚 Documentation

• Added comprehensive examples and troubleshooting guide in examples/README.md
• Documented VNet peering scenarios and prerequisites
• Included complete configuration examples for different use cases

🔒 Security & Networking

• All database connectivity uses private endpoints (no public access)
• DNS resolution handled through existing private DNS zones
• Network traffic flows through VNet peering for secure communication

✅ Backward Compatibility

All existing functionality remains unchanged. The new features are opt-in via the use_existing_database flag (defaults to false).