feat: support dual-stack (IPv4 + IPv6) clusters for self-hosted nodes

[MarcPow]

Description

Provision dual-stack (IPv4 + IPv6) nodes on dual-stack Azure CNI clusters.

Previously, nodes launched by Karpenter on a dual-stack cluster were IPv4-only: the NIC had only a primary IPv4 IP configuration, IPv6 load-balancer backend pools were never attached, and the bootstrap contract's IPv6 dual-stack flag was never set. This effectively made dual-stack clusters unsupported for the self-hosted (VM-direct) provisioning path. This PR wires the cluster's IP families through to node provisioning so each node receives both an IPv4 and an IPv6 address, mirroring how AKS configures dual-stack VMSS nodes.

What this PR does

• Option: add a NodeIPFamilies option (env NODE_IP_FAMILIES, default IPv4) and an IsIPv6Enabled() helper. The deploy script (configure-values.sh) populates it from the cluster's networkProfile.ipFamilies, threaded through the Helm values template.
• LoadBalancer provider: classify backend pools by IP version and populate BackendAddressPools.IPv6PoolIDs from the well-known IPv6 inbound/outbound pools (previously discarded).
• VM NIC (VM-direct path): on dual-stack clusters, append a single non-primary ipv6config IP configuration (dynamic, IPv6) with the IPv6 LB backend pools attached. A NIC may only have one IPv6 ip config, so it's added once, outside the secondary-IP loop. Mirrors the AKS RP node NIC construction.
• Bootstrap: set the existing (previously unused) IPv6DualStackEnabled node bootstrap variable from the cluster IP families.
• Docs: scope the README "IPv6 clusters" limitation to single-stack IPv6 only.

Testing

• Unit tests: IsIPv6Enabled; LoadBalancer IPv6 pool classification; the IPv6 NIC ip configuration on dual-stack vs single-stack clusters.
• Live validation on a real dual-stack Azure CNI Overlay cluster (ipFamilies: [ipv4, ipv6]):
  • configure-values.sh correctly auto-extracted NODE_IP_FAMILIES=ipv4,ipv6.
  • A Karpenter-provisioned node's NIC came up with ipconfig (IPv4, primary, 10.224.0.6) and ipv6config (IPv6, non-primary, fdd2:…::6) — matching the system VMSS reference NIC exactly.
  • The node registered both IPv4 and IPv6 InternalIP addresses, and workloads scheduled and ran.

Scope & notes

• Covers the self-hosted, VM-direct provisioning path (aksscriptless / bootstrappingclient).
• The AKS Machine API path (aksmachineapi / NAP) is unchanged: the AKS RP owns NIC and LB construction there, and the Machine API has no per-machine IP-family field. Dual-stack for NAP is currently gated off at the AKS RP and is tracked as separate RP work.
• Single-stack IPv6-only clusters remain unsupported.