Conditionally creates billing sink if the billing account is of type resource (#3130)

* updated billing.tf file to conditionally creates billing sink if the billing account resides out of org

* replace resources with modules

* replace individual iam resources with billing_iam local passed in the module

* update module name and path and move iam from local to module call

* update README.md

* Add log_bucket option to billing_account variable, and update billing account IAM assignments

* update 0-bootstrap README to reflect changes to billing account module

* Update current bootstrap tests to reflect the change to billing_account variable

* Create test for the case when billing account log bucket is created

* running fmt

---------

Co-authored-by: Ludovico Magnocavallo <ludomagno@google.com>

tests/fast/stages/s0_bootstrap/cicd.yaml

@@ -2493,6 +2493,7 @@ outputs:
       force_create:
         dataset: false
         project: false
+        log_bucket: false
       id: 000000-111111-222222
       is_org_level: true
       no_iam: false

tests/fast/stages/s0_bootstrap/external_billing_account.tfvars

@@ -0,0 +1,23 @@
+billing_account = {
+  id           = "000000-111111-222222"
+  is_org_level = false
+  force_create = {
+    dataset    = true
+    project    = true
+    log_bucket = true
+  }
+}
+essential_contacts = "gcp-organization-admins@fast.example.com"
+groups = {
+  gcp-support = "group:gcp-support@example.com"
+}
+org_policies_config = {
+  import_defaults = false
+}
+organization = {
+  domain      = "fast.example.com"
+  id          = 123456789012
+  customer_id = "C00000000"
+}
+outputs_location = "/fast-config"
+prefix           = "fast"

tests/fast/stages/s0_bootstrap/simple.yaml

@@ -1604,6 +1604,7 @@ outputs:
       force_create:
         dataset: false
         project: false
+        log_bucket: false
       id: 000000-111111-222222
       is_org_level: true
       no_iam: false

tests/fast/stages/s0_bootstrap/tftest.yaml

@@ -23,5 +23,8 @@ tests:
     inventory:
       - simple.yaml
       - managed_org_policies.yaml
+  external_billing_account:
+    inventory:
+      - external_billing_account.yaml
   iam_by_principals:
   cicd: