Compute VM module refactor (#3805)

* add ad for compute-vm refactor

* Exclue nic_type from validated fields, add split of main.tf and template.tf

* boot disk

* fix examples and fixtures

* attached disks

* fix further examples and module-level tests

* remove extra file

* fix mig examples

* finish refactoring variables

* align fast and other modules

* refactor(compute-vm): align examples and ADR with the newly implemented interface

This commit addresses the remaining references of the `instance_type` and `confidential_compute` parameters in the testing environment and updates the ADR.

* feat(compute-vm): add network_performance_config to instance and templates

This change implements the usage of the `network_performance_tier` variable we added earlier into the actual Terraform resources.

---------

Co-authored-by: Wiktor Niesiobędzki <wiktorn@google.com>

modules/net-lb-ext/README.md

@@ -159,9 +159,11 @@ module "instance-group" {
   }]
   boot_disk = {
     initialize_params = {
+      type = "pd-ssd"
+      size = 10
+    }
+    source = {
       image = "projects/cos-cloud/global/images/family/cos-stable"
-      type  = "pd-ssd"
-      size  = 10
     }
   }
   tags = ["http-server", "ssh"]
@@ -196,6 +198,7 @@ module "nlb" {
 ```
 
 ## Deploying changes to load balancer configurations
+
 For deploying changes to load balancer configuration please refer to [net-lb-app-ext README.md](../net-lb-app-ext/README.md#deploying-changes-to-load-balancer-configurations)
 <!-- BEGIN TFDOC -->
 ## Variables