Add Dual Stack Public ECR endpoint support

[BDworak]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Adds the dual stack ECR endpoint ecr-public.aws.com to ecr-credential-provider so that it can provide authorization for the public registry. Tests updated to test both possible variants of this change.

Special notes for your reviewer:
Related to:
#1069
awslabs/amazon-eks-ami#2082

Does this PR introduce a user-facing change?:
NONE

Add ECR dual stack public endpoint support to ecr-credential-provider

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

This issue is currently awaiting triage.

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[k8s-ci-robot]

Hi @BDworak. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[cartermckinnon]

/ok-to-test

[mmerkes]

/lgtm