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>

tests/modules/compute_vm/context-template-regional.tfvars

@@ -1,8 +1,6 @@
-attached_disks = [{
-  name = "data-0"
-  size = 10
-  }
-]
+attached_disks = {
+  data-0 = {}
+}
 context = {
   addresses = {
     ext-test-0 = "35.10.10.10"