SubnetGatewayIPV6Address and IPv6SubnetCIDRBlock metadata for IPv6-only tasks

[amogh09]

Summary

This PR makes the following two changes to task metadata returned for IPv6-only awsvpc mode tasks.

1. A new SubnetGatewayIPV6Address field will be returned. Its value will be the subnet gateway IPv6 address used to create the default route for the task.
2. Existing IPv6SubnetCIDRBlock field will be computed based on the actual subnet prefix length instead of the hardcoded /64 that is used for dual-stack tasks today.

Testing

IPv6-only subnet with /64 prefix -

ip-10-0-0-23 ❱ docker exec 6759f9efa687 sh -c 'curl -s $ECS_CONTAINER_METADATA_URI_V4' | jq '.Networks'
[
  {
    "NetworkMode": "awsvpc",
    "IPv6Addresses": [
      "2600:1f14:323a:e000:9233:2126:1e0f:ced1"
    ],
    "AttachmentIndex": 0,
    "MACAddress": "02:f2:3d:fe:5d:bb",
    "IPv6SubnetCIDRBlock": "2600:1f14:323a:e000::/64",
    "DomainNameServers": [
      "fd00:ec2::253"
    ],
    "DomainNameSearchList": [
      "us-west-2.compute.internal"
    ],
    "SubnetGatewayIPV6Address": "2600:1f14:323a:e000::1/64"
  }
]

IPv6-only subnet with /60 prefix -

ip-10-0-0-23 ❱ docker exec 6e086c5276d2 sh -c 'curl -s $ECS_CONTAINER_METADATA_URI_V4' | jq '.Networks'
[
  {
    "NetworkMode": "awsvpc",
    "IPv6Addresses": [
      "2600:1f14:323a:e014:588a:9986:158:b9ba"
    ],
    "AttachmentIndex": 0,
    "MACAddress": "02:5b:44:5e:c7:ef",
    "IPv6SubnetCIDRBlock": "2600:1f14:323a:e010::/60",
    "DomainNameServers": [
      "fd00:ec2::253"
    ],
    "DomainNameSearchList": [
      "us-west-2.compute.internal"
    ],
    "SubnetGatewayIPV6Address": "2600:1f14:323a:e010::1/60"
  }
]

Dual-stack subnet with /60 prefix (prefix gets hardcoded as /64 and SubnetGatewayIPv6Address is not populated) -

ip-10-0-0-23 ❱ docker exec 234da5144aea sh -c 'curl -s $ECS_CONTAINER_METADATA_URI_V4' | jq '.Networks'
[
  {
    "NetworkMode": "awsvpc",
    "IPv4Addresses": [
      "10.0.5.147"
    ],
    "IPv6Addresses": [
      "2600:1f14:323a:e023:170:b7b:3c6d:9a82"
    ],
    "AttachmentIndex": 0,
    "MACAddress": "02:b1:6e:6e:d3:17",
    "IPv4SubnetCIDRBlock": "10.0.5.0/24",
    "IPv6SubnetCIDRBlock": "2600:1f14:323a:e023::/64",
    "PrivateDNSName": "ip-10-0-5-147.us-west-2.compute.internal",
    "SubnetGatewayIpv4Address": "10.0.5.1/24"
  }
]

New tests cover the changes: yes

Description for the changelog

Enhancement: SubnetGatewayIPV6Address and IPv6SubnetCIDRBlock metadata for IPv6-only tasks

Additional Information

Does this PR include breaking model changes? If so, Have you added transformation functions?

No

Does this PR include the addition of new environment variables in the README?

No

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.