docs(rfc): add rfc for vm lifecycle extensions

[cheese-head]

Summary

Adds RFC 0004, which proposes extending the VM driver with runtime backend selection and an in-process lifecycle extension API. The design keeps libkrun as the default while allowing QEMU selection and future VM-driver-managed attachments such as VFIO, vDPA, DPU-backed NICs, vTPMs, and audit hooks.

Related Issue

N/A

Changes

• Defines template.runtime_class_name for selecting libkrun, qemu, or the default VM backend.
• Introduces the VmLifecycleExtension API for launch, rollback, delete, and restart reconcile hooks.
• Specifies extension state isolation, attachment handling, launcher argument validation, reconcile modes, operator configuration, and observability requirements.
• Clarifies compatibility expectations and the boundary between user-facing resource requests and VM-driver internal realization.

Testing

Documentation-only RFC change.

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.