add support for Firecracker microVMs

We're excited about [Firecracker](https://github.com/firecracker-microvm/firecracker) and the ability to provide secure, multi-tenant, minimal-overhead execution of container workloads.

We need to investigate the best way to integrate it, and decide whether we can include it in the Kubernetes variant, or whether it needs its own variant.

Some areas for research:
- [ ] [firecracker-containerd](https://github.com/firecracker-microvm/firecracker-containerd) and how it relates to containerd
- [ ] conformance status for CRI and Kubernetes
- [ ] support for primary processor types (Intel, AMD, ARM)

We would want the microVM kernel and userspace to also be Bottlerocket, and may need a simplified boot path for this use case.