fix(stack): introduce configsubscription variable

[jta]

This commit addresses a limitation of our current module API whereby passing a bucket reference would raise an error, e.g.:

module "forwarder" {
  ...
  config_delivery_bucket_name = aws_s3_bucket.this.id
}

would trigger:

│ Error: Invalid count argument
│
│   on .terraform/modules/forwarder/modules/stack/configsubscription.tf line 2, in module "configsubscription":
│    2:   count  = var.config_delivery_bucket_name != null ? 1 : 0
│
│ The "count" value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created. To work around this, use the -target argument to
│ first apply only the resources that the count depends on.

To work around this, we can instead gate the condition on the presence of an object, which is the pattern we follow for all other nested modules:

module "forwarder" {
  ...
  configsubscription = {
    delivery_bucket_name = aws_s3_bucket.this.id
  }
}

In this case, we no longer need to worry about whether delivery_bucket_name is known, since our condition will verify the presence of configsubscription instead.

[obs-gh-abhishekrao]

@jta I see we pick up tests on this repo. Can you add one for these changes? We should have caught the error this fixes through one of those
https://github.com/observeinc/terraform-aws-collection/blob/main/examples/simple/tests/simple.tftest.hcl

[jta]

I can extend the tests, it just doesn't really cover the bug because in .tftest.hcl we create a bucket in a separate run block. Each block has its own terraform state, so we don't actually get an end-to-end apply upfront.

[obs-gh-abhishekrao]

I can extend the tests, it just doesn't really cover the bug because in .tftest.hcl we create a bucket in a separate run block. Each block has its own terraform state, so we don't actually get an end-to-end apply upfront.

Got it. In any case we might need to add tests for the filedrop approach if we don't already have one so I'll leave it to you whether it should happen in this PR or a different one.