Firecracker backend PoC for function execution

[ashupednekar]

Summary

Add an experimental Firecracker backend PoC for running functions in microVMs.

Motivation

MicroVM isolation can improve tenant separation while keeping startup latency practical for bursty workloads.

Proposal

• Add a feature-gated firecracker executor path.
• Build OCI/rootfs artifact to boot a minimal runtime guest.
• Map function invoke request/response over vsock or unix socket bridge.
• Integrate with existing lease/activation semantics (Activate, keep-warm, deprovision).

Acceptance criteria

• Feature flag to enable Firecracker backend in non-production mode.
• End-to-end invoke works for at least one language runtime.
• Documented limitations (cold start, image format, host requirements).