[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

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