Support AAAA records for IPv6 DNS support

[mikereinhold]

Copilot currently only supports IPv4 DNS aliases ('A') in the [workload custom domain cf-custom-resource0(https://github.com/aws/copilot-cli/blob/mainline/cf-custom-resources/lib/wkld-custom-domain.js#L280).

Copilot needs to support IPv6 DNS aliases ('AAAA') as well. Initially this could be both on by default or with options to enable/disable.

[bvtujo]

This also makes a lot of sense as a feature request. We'll have to investigate what changes need to be made under the hood, as we do lots of DNS delegation and redirection to ALB and CF DNS.