Add subnet into existing default VPC with Terraform

1. Define data source to retrieve an existing default AWS VPC:

   data "aws_vpc" "existing_vpc" {
     default = true
   }

2. Declare an AWS subnet resource named dev-subnet.
   This subnet is associated with the retrieved existing VPC and configured with a specific CIDR block 172.31.48.0/20:

   resource "aws_subnet" "dev-subnet" {
     vpc_id = data.aws_vpc.existing_vpc.id
     cidr_block = "172.31.48.0/20"
     availability_zone = "eu-central-1a"
   }