feat(datasource/aliyun): add IPv6 metaserver and DHCP strategy based on NIC metadata

[challvy]

Proposed Commit Message

Fixes #6892

feat(aliyun): support IPv6 metaserver and IPv6-only NIC config

Alibaba Cloud ECS now exposes the metadata service over both IPv4
(100.100.100.200) and IPv6 (fd00:100::100:200), and supports attaching
IPv6-only ENIs that have no IPv4 address. The current AliYun datasource
only configures the IPv4 endpoint and unconditionally emits dhcp4: true
for every NIC, which prevents IPv6-only instances from reaching IMDS
and causes long boot delays on IPv6-only ENIs.

This change:

- Adds the IPv6 metadata endpoint to DataSourceAliYun.metadata_urls so
  wait_for_url tries both endpoints. IPv4 instances are unaffected.
- Makes DHCP rendering metadata-driven in
  convert_ecs_metadata_network_config():
    * dhcp6 is enabled when 'ipv6s' is present on a NIC (existing).
    * dhcp4 is disabled and dhcp4-overrides is dropped when
      'private-ipv4s' is absent on a NIC, so DHCPv4 is not attempted
      on IPv6-only NICs.
- Adds unit tests covering IPv4-only, dual-stack, and IPv6-only NIC
  scenarios under tests/unittests/sources/test_aliyun.py.

Additional Context

The IPv6 metadata endpoint and IPv6-only ENIs are documented features
of Alibaba Cloud ECS. With these changes, cloud-init can correctly
fetch metadata and render network configuration on:

NIC metadata	dhcp4	dhcp6
private-ipv4s only	true	false
private-ipv4s + ipv6s (dual-stack)	true	true
ipv6s only (IPv6-only ENI)	false	true

Backwards compatibility:

• Existing IPv4-only instances are unaffected — their NIC metadata
  always contains private-ipv4s, so dhcp4 stays true.
• Adding a second URL to metadata_urls does not change behaviour
  when the IPv4 IMDS endpoint responds successfully.

Test Steps

1. Unit tests (covers all three NIC scenarios):

   tox -e py3 -- tests/unittests/sources/test_aliyun.py
   

   Expected: all tests pass, including the new
   test_dhcp4_disabled_when_no_private_ipv4s,
   test_dhcp6_enabled_when_ipv6s_present, and
   test_ipv6_only_nic_config.

2. Live verification on an Alibaba Cloud ECS instance:

   a. Launch a dual-stack ECS instance with both IPv4 and IPv6
   addresses, boot with this cloud-init build, and confirm:

   • /run/cloud-init/instance-data.json contains metadata fetched
     from IMDS.
   • The rendered netplan contains dhcp4: true and dhcp6: true
     for the dual-stack NIC.

   b. Attach an IPv6-only secondary ENI (no private-ipv4s in
   metadata), reboot, and confirm:

   • Boot is not delayed by DHCPv4 timeouts on the IPv6-only NIC.
   • The rendered netplan for that NIC contains dhcp4: false and
     dhcp6: true.

   c. (Optional) Disable IPv4 IMDS on a test instance and confirm
   cloud-init still reaches metadata via
   http://[fd00:100::100:200].

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)