feat: Allow the overwrite of subnets for Fargate profiles (#1117)

NOTES: The EKS cluster can be provisioned with both private and public subnets. But Fargate only accepts private ones. This new variable allows to override the subnets to explicitly pass the private subnets to Fargate and work around that issue.

examples/fargate/main.tf

@@ -100,6 +100,9 @@ module "eks" {
       #   GithubOrg   = "terraform-aws-modules"
       # }
 
+      # using specific subnets instead of all the ones configured in eks
+      # subnets = ["subnet-0ca3e3d1234a56c78"]
+
       tags = {
         Owner = "test"
       }