feat: relax cpu_options schema and add amd_sev_snp + nested_virtualization support (#5039)

### Summary
This PR updates the module’s CPU options plumbing to support newer EC2
Launch Template `cpu_options` fields (notably `nested_virtualization`
and `amd_sev_snp`) while also making CPU topology settings optional so
users aren’t forced to specify `core_count`/`threads_per_core` just to
enable other CPU options.

### Motivation / Problem
When configuring runners via `multi-runner`, the `cpu_options` object
currently requires `core_count` and `threads_per_core`. This blocks
use-cases where users want to set only newly-supported provider fields
like `nested_virtualization` without also pinning CPU topology. AWS
itself treats these settings as optional and instance-type-dependent.

### What changed
- **Relaxed the `cpu_options` object type** so these fields are
optional:
  - `core_count`
  - `threads_per_core`
- **Added optional support for:**
  - `nested_virtualization` (valid: `enabled` / `disabled`)
  - `amd_sev_snp` (valid: `enabled` / `disabled`)
- **Added validation** on `amd_sev_snp` and `nested_virtualization` to
restrict values to `enabled|disabled` when set.
- **Launch template wiring:** updated the `aws_launch_template`
`cpu_options` block to pass through `amd_sev_snp` and
`nested_virtualization` (using `try(..., null)` so unset values remain
unset).
- **Multi-runner schema updated** so per-runner `cpu_options` can
include the new fields and no longer forces
`core_count`/`threads_per_core`.

### Compatibility / Notes
- Existing configurations that set `core_count` and `threads_per_core`
continue to work.
- Users can now set `nested_virtualization` or `amd_sev_snp` without
specifying CPU topology.
- AWS will still enforce instance-type support at API time (e.g., nested
virt only on supported 8th-gen Intel families; SEV-SNP only on supported
AMD families).

### Testing
- `terraform init`
- `terraform validate`
- `terraform plan` with `cpu_options.nested_virtualization = "enabled"`
on supported instance families

```
  instance_types:
    - c8i.xlarge
    - m8i.xlarge
    - r8i.xlarge
  cpu_options:
    nested_virtualization: enabled
```

Fixes #5040

---------

Signed-off-by: Brian Long <brilong@cisco.com>
Co-authored-by: Brend Smits <brend.smits@philips.com>

Author: brilong

examples/default/.terraform.lock.hcl

@@ -2,7 +2,7 @@ terraform {
   required_providers {
     aws = {
       source  = "hashicorp/aws"
-      version = ">= 6.21"
+      version = ">= 6.33"
     }
     local = {
       source  = "hashicorp/local"

examples/default/versions.tf

@@ -2,7 +2,7 @@ terraform {
   required_providers {
     aws = {
       source  = "hashicorp/aws"
-      version = ">= 6.21"
+      version = ">= 6.33"
     }
     local = {
       source  = "hashicorp/local"