Annotate pods with IPv6 details from branch ENI when available

[jdn5126]

Issue #, if available:
N/A

Description of changes:
In order to support Security Groups for Pods in IPv6 clusters, the VPC Resource Controller must create trunk and branch ENIs with IPv6 addresses assigned. This happens already based on the subnet used in an IPv6 cluster, so VPC Resource Controller's next role is to annotate pods with branch ENI information.

This PR annotates pods with the IPv6 address and CIDR that is configured for the branch ENI, if there is one. The reason that IPv6 fields are added in-addition, instead of in-place is two-fold:

1. VPC Resource Controller must support older VPC CNI versions, so it cannot change existing JSON names.
2. If dual-stack is eventually supported, the pod will need IPv4 and IPv6 fields, so this change facilitates that.

I tested this code against an IPv4 and an IPv6 cluster to verify that it works. In an IPv4 cluster, the annotation looks as follows:

k get pod nginx-deployment-6c4d879f6c-7ktc5 -n sg-test -o jsonpath='{.metadata.annotations}'
{"vpc.amazonaws.com/pod-eni":"[{\"eniId\":\"eni-0f11cb35d7d5bf4d6\",\"ifAddress\":\"0e:ee:16:bd:41:31\",\"privateIp\":\"192.168.19.148\",\"ipv6Addr\":\"\",\"vlanId\":1,\"subnetCidr\":\"192.16
8.0.0/19\",\"subnetV6Cidr\":\"\"}]"}

In an IPv6 cluster, it looks as follows:

{"vpc.amazonaws.com/pod-eni":"[{\"eniId\":\"eni-0f11cb35d7d5bf4d6\",\"ifAddress\":\"0e:ee:16:bd:41:31\",\"privateIp\":\"192.168.19.148\",\"ipv6Addr\":\"2600::\",\"vlanId\":1,\"subnetCidr\":\"192.16
8.0.0/19\",\"subnetV6Cidr\":\"2600::/64\"}]"}

There are no backward-incompatible changes here, but I am still awaiting results from integration tests for further coverage.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[sushrk]

Changes LGTM overall, thanks @jdn5126! I have kicked off integration tests on Gitlab and will update the results.

EDIT: Seeing errors in setup stage, re-running this.

[haouc]

lgtm