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>

tests/fixtures/compute-mig.tf

@@ -25,7 +25,7 @@ module "_instance-template" {
     addresses  = null
   }]
   boot_disk = {
-    initialize_params = {
+    source = {
       image = "projects/cos-cloud/global/images/family/cos-stable"
     }
   }

tests/fixtures/compute-vm-group-bc.tf

@@ -23,7 +23,7 @@ module "compute-vm-group-b" {
     subnetwork = var.subnet.self_link
   }]
   boot_disk = {
-    initialize_params = {
+    source = {
       image = "cos-cloud/cos-stable"
     }
   }
@@ -40,7 +40,7 @@ module "compute-vm-group-c" {
     subnetwork = var.subnet.self_link
   }]
   boot_disk = {
-    initialize_params = {
+    source = {
       image = "cos-cloud/cos-stable"
     }
   }