add dualstack support for AWS provider with ALB ingress controllers

[twilfong]

This PR adds dualstack support for ALB ingress, and fixes issue #520.

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[twilfong]

/assign @linki

[njuettner]

Another question: Is this cleaned up correctly? When you delete the ingress again, does it clean up both records?

[twilfong]

Another question: Is this cleaned up correctly? When you delete the ingress again, does it clean up both records?

Yes it does delete both records. Any change (Create, Upsert, or Delete) that goes through the AWS provider will now check for the presence of a dualstack label (with value = "true") in the registry entry for the given endoint (e.g. the TXT record if using TXT registry) if the endpoint returns true for isAWSLoadBalancer(). In the case where these two things are true, the single change of type A is turned into two changes, both A and AAAA. This works for creates, deletes, or upserts.

I modified the TestAWSCreateRecordsWithALIAS case in aws_test.go to include a dualstack loadbalancer. But, we might want to add cases that test delete and update. I did run manual tests to ensure that this works, (deletes and updates,) and the codepath definitely is the same for any kind change. I'm not sure how much extra value would be added by specific test cases for dualstack endpoint upsert/delete, but they would be simple enough to add to aws_test.go if we want them.

[njuettner]

/lgtm
/approve

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This pull-request has been approved by: njuettner

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[autarchprinceps]

Does this work for dualstack NLB on K8s services as well? For example if you use a traefik or nginx or similar load balancer, but with NLB, external-dns enters that as the A record.