Added traffic.istio.io/ingress-public-address annotation

[schurcht]

Adding a service annotation that allows users to override the IP Istio will use to reach out to gateways. This enables you to use a private load balancer with a public IP that DNATs to it.

[istio-testing]

Hi @schurcht. Thanks for your PR.

I'm waiting for a istio 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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[keithmattix]

I strongly suspect this is with respect to istio/istio#55041

[schurcht]

What is this actually intended to be used by?

https://github.com/istio/istio/issues/55041

[hzxuzhonghu]

not needed