feat: add support for security group referencing to transit-gateway module

AWS recently introduced security group referencing for Transit Gateway VPC attachments. This feature was available in the standalone resource `aws_ec2_transit_gateway_vpc_attachment` but not in the transit-gateway module. Added support for it in the module.

Author: syedimranhassan

examples/complete/main.tf

@@ -34,10 +34,11 @@ module "tgw" {
 
   vpc_attachments = {
     vpc1 = {
-      vpc_id       = module.vpc1.vpc_id
-      subnet_ids   = module.vpc1.private_subnets
-      dns_support  = true
-      ipv6_support = true
+      vpc_id                             = module.vpc1.vpc_id
+      subnet_ids                         = module.vpc1.private_subnets
+      security_group_referencing_support = true
+      dns_support                        = true
+      ipv6_support                       = true
 
       transit_gateway_default_route_table_association = false
       transit_gateway_default_route_table_propagation = false

main.tf

@@ -73,6 +73,7 @@ resource "aws_ec2_transit_gateway_vpc_attachment" "this" {
   dns_support                                     = try(each.value.dns_support, true) ? "enable" : "disable"
   ipv6_support                                    = try(each.value.ipv6_support, false) ? "enable" : "disable"
   appliance_mode_support                          = try(each.value.appliance_mode_support, false) ? "enable" : "disable"
+  security_group_referencing_support              = try(each.value.security_group_referencing_support, false) ? "enable" : "disable"
   transit_gateway_default_route_table_association = try(each.value.transit_gateway_default_route_table_association, true)
   transit_gateway_default_route_table_propagation = try(each.value.transit_gateway_default_route_table_propagation, true)