net: add configurable nat66Prefix for shared networking

[stek29]

Add network.nat66Prefix and network-nat66-prefix for vmnet non-VZ shared networking, while preserving the existing behavior of not passing any value for nat66-prefix to vmnet by default.

This allows creating multiple colima instances with shared vmnet using IPv6 and is a follow-up for PR #1628, which fixed IPv4 use-case.

LLM disclosure: I've used LLM based coding Agent for spinning up test VMs before/after this change to test different options. None of the code (including tests) was written by an LLM.

[stek29]

example reproducer:

for n in 1 2; do
  colima start "shared-noprefix-v6-$n" \
    --vm-type qemu --network-address \
    --network-subnet "192.168.11$n.0/24" \
    # --network-nat66-prefix "fd22:cafe:babe:f00${n}::"
done

for n in 1 2; do
  colima ssh --profile "shared-noprefix-v6-$n" -- \
    curl -v --max-time 5 "https://[2606:4700:4700::1111]/"
done

here second VM's IPv6 will time out without the network-nat66-prefix fix.

[abiosoft]

Is nat66 a convention? I would expect the wording simply include ipv6.

[stek29]

@abiosoft it's the name of the option in vmnet: https://developer.apple.com/documentation/vmnet/vmnet_nat66_prefix_key

[abiosoft]

Kindly rebase.

[stek29]

@abiosoft rebased, but actions seem to be failing because of a billing issue.

[abiosoft]

actions seem to be failing because of a billing issue.

Yeah, it was due to exceeding budget of copilot usage and an unusual rejection of a previously valid payment card.
It should get resolved within the next week.

[majidamomajidam]

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