fix: Merge Managed and Unmanaged Users

- The input variable `existing_sso_users`, while present, was
  previsously unused.
  This includes it into a new merged collection that then
  updates managed as well as unmanaged user memberships.

Author: alexandertgtalbot

locals.tf

@@ -1,10 +1,13 @@
 # - Users and Groups -
 locals {
-  # Create a new local variable by flattening the complex type given in the variable "sso_users"
+  # To support both managed and federated identities, let's combine the two user collections.
+  sso_users = merge(var.sso_users, var.existing_sso_users)
+
+  # Create a new local variable by flattening the complex type given in the local aggregated variable "sso_users"
   flatten_user_data = flatten([
-    for this_user in keys(var.sso_users) : [
-      for group in var.sso_users[this_user].group_membership : {
-        user_name  = var.sso_users[this_user].user_name
+    for this_user in keys(local.sso_users) : [
+      for group in local.sso_users[this_user].group_membership : {
+        user_name  = local.sso_users[this_user].user_name
         group_name = group
       }
     ]