feat(compute_instance): add standalone VM support without instance template

[AryamanParida]

Problem

The modules/compute_instance module only supports
google_compute_instance_from_template, which requires a mandatory
instance_template. This means the module cannot manage standalone
VMs that were created directly (without a template) — there is no
way to import such instances into this module's state because
google_compute_instance_from_template and google_compute_instance
are completely different Terraform resource types.

Solution

Make instance_template optional (default = null). When null:

• A google_compute_instance resource is created/managed instead.
• New variables cover the required standalone configuration:
  machine_type, boot_disk, service_account, metadata,
  tags, and scheduling.

When instance_template is provided (the existing default), the
module behaves exactly as before — fully backward-compatible.

Changes

file	what changed
variables.tf	instance_template default set to null; 7 new optional variables added with null/empty defaults
main.tf	google_compute_instance_from_template gated on instance_template != null; new google_compute_instance resource added for standalone path
outputs.tf	All outputs coalesced via ternary on var.instance_template != null

New variables (all optional, backward-compatible defaults)

variable	type	description
machine_type	string	Machine type for standalone VMs
boot_disk	object	Boot disk config (source_image, disk_size_gb, disk_type, disk_labels, kms_key_self_link, auto_delete, device_name)
service_account	object	Service account (email + scopes)
metadata	map(string)	Instance metadata
tags	list(string)	Network tags
scheduling	object	on_host_maintenance, automatic_restart, preemptible

Usage (new standalone path)

module "vm" {
  source  = "terraform-google-modules/vm/google//modules/compute_instance"

  project_id   = "my-project"
  region       = "us-central1"
  zone         = "us-central1-a"
  hostname     = "my-vm"
  network      = "projects/my-project/global/networks/default"
  subnetwork   = "projects/my-project/regions/us-central1/subnetworks/default"

  machine_type = "n2-standard-4"
  boot_disk = {
    source_image = "debian-cloud/debian-12"
    disk_size_gb = 50
    disk_type    = "pd-balanced"
  }

  service_account = {
    scopes = ["cloud-platform"]
  }
}

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