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.

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

modules/net-lb-app-ext/README.md

@@ -311,12 +311,14 @@ module "win-template" {
   project_id      = var.project_id
   zone            = "${var.region}-a"
   name            = "win-template"
-  instance_type   = "n2d-standard-2"
+  machine_type    = "n2d-standard-2"
   create_template = {}
   boot_disk = {
     initialize_params = {
+      size = 70
+    }
+    source = {
       image = "projects/windows-cloud/global/images/windows-server-2019-dc-v20221214"
-      size  = 70
     }
   }
   network_interfaces = [{