feat: relax cpu_options schema and add amd_sev_snp + nested_virtualization support

[brilong]

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

[brilong]

 aws ec2 describe-launch-template-versions \                                                                                                                                                               
  --launch-template-name u24-nv-xl-amd64-runner \
  --versions '$Latest' \
  --query 'LaunchTemplateVersions[0].LaunchTemplateData.CpuOptions' \
  --output json
{
    "NestedVirtualization": "enabled"
}