[Feature Request] Userspace networking (passt/slirp)

# Feature Request

Allow firecracker to delegate VM networking to a userspace process. This can be highly beneficial in low privilege scenarios where `CAP_NET_ADMIN` is not available and makes VM config a lot faster (no iptables/nftables work required)
I would love to discuss any blockers you think might be present that may prevent this feature from being feasible
If there's not any, i'd be happy to work on a PR!

## Describe possible alternatives

N/A

## Additional context

N/A

## Checks

- [x] Have you searched the Firecracker Issues database for similar requests?
- [x] Have you read all the existing relevant Firecracker documentation?
- [x] Have you read and understood Firecracker's core tenets?