Add Hyperdisk support for GCE machine types

[rluisr]

what

• Added automatic detection for Hyperdisk-only machine families (C4D, H4D, X4, M4, A3 Ultra, A3 Mega)
• When Hyperdisk-only machine types are detected, both boot and persistent disks automatically default to hyperdisk-balanced
• Added new boot_disk_type variable to allow explicit override of boot disk type
• Added validation to persistent_disk_type to prevent using pd-* disk types with Hyperdisk-only machines
• Added validation to machine_type to prevent using ARM64 machine types (C4A, A4X, A4, T2A, T2D)
• Updated README.md with new "Hyperdisk Support" section and FAQ entry
• Updated variable documentation to reflect Hyperdisk requirements
• Updated documentation to clarify that ARM64 machine types are not supported

why

• Google Cloud requires Hyperdisk storage for specific machine series (C4D, H4D, X4, M4, A3 Ultra, A3 Mega)
• Without this change, users attempting to use these machine types would encounter deployment errors due to incompatible disk types
• Automatic detection eliminates the need for manual disk type configuration while still allowing overrides when needed
• Validation helps catch configuration errors early in the Terraform plan phase rather than during apply
• ARM64 machine types (C4A, A4X, A4, T2A, T2D) are not supported because Atlantis does not run on ARM64 architecture
• Even though some ARM64 machine types require Hyperdisk, they cannot be used with this module due to Atlantis' x86-64 requirement
• This enhancement makes the module more user-friendly and prevents common configuration mistakes

references

• This change is based on Google Cloud's Hyperdisk requirements for newer machine families
• Related documentation: https://cloud.google.com/compute/docs/disks/hyperdisks
• Addresses the requirement that certain machine types cannot use traditional persistent disks (pd-standard, pd-ssd, pd-balanced)
• Note: ARM64 + Hyperdisk combinations remain unsupported due to Atlantis architecture requirements

[rluisr]

This PR needs to be marked as breaking change (since you can't go from n2 -> c4a).

[rluisr]

I couldn't boot into COS on ARM.
Anyone with expertise, please try deploying Atlantis and investigating the issue as follows:

module "atlantis" {
  #source       = "runatlantis/atlantis/gce"
  source               = "git::https://github.com/rluisr/terraform-gce-atlantis.git?ref=feature/hyperdisk-support"
  name                 = "atlantis"
  image                = "ghcr.io/runatlantis/atlantis:v0.34.0"
  domain               = local.atlantis_domain
  project              = data.google_project.project.project_id
  network              = data.google_compute_network.default.id
  subnetwork           = data.google_compute_subnetwork.default.id
  region               = "asia-northeast1"
  zone                 = "asia-northeast1-b"
  machine_type         = "c4a-highcpu-4"
  persistent_disk_type = "hyperdisk-balanced"
}

[rluisr]

x86/64 machine + hyperdisk is working perfectly.

[rluisr]

hi @Zurvarian, can you take a look? Let me know if you have any suggestions and whether this works for you 👍