Dualstack Support for the AWS SD provider

[mjlshen]

What would you like to be added:
In #520 and #1079, the AWS provider gained the ability to create both A and AAAA records for dualstack load balancers. This same functionality currently does not exist in the AWS-SD provider as any LoadBalancer endpoint will result in the creation of only an A record:

external-dns/provider/awssd/aws_sd.go

Lines 488 to 493 in b834fef

	if ep.RecordType == endpoint.RecordTypeCNAME {
	if p.isAWSLoadBalancer(target) {
	attr[sdInstanceAttrAlias] = target
	} else {
	attr[sdInstanceAttrCname] = target
	}

Why is this needed:
This is a parity item with the AWS provider and I noticed this while working on #4721 to add initial IPv6 support to the AWS-SD provider.

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[ivankatliarchuk]

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/area provider/aws

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