add support for zone/subzone level failover settings

[masquee]

Current failover settings only support failover from one region to another region, but sometimes we prefer to failover from one zone to another zone, even from one subzone to another subzone.

This pull request adds support for zone/subzone failover settings. For example, failover settings can set as { From: "region1/zone1/subzone1", To: "region2/zone2/subzone2" }

For pepole who only concerns about region-level failover, they can still use failover setting as follow, which compatible with before. { From: "region1", To: "region2" }

This pull request just modify failover releated comments, no proto struct changes.

Related implementation changes resides in this pull request: https://github.com/istio/istio/pull/30781

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[istio-testing]

Hi @masquee. 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.

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[istio-testing]

@masquee: PR needs rebase.

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[hzxuzhonghu]

@masquee See a more general way, supporting failover by topology keys

[istio-policy-bot]

🚧 This issue or pull request has been closed due to not having had activity from an Istio team member since 2021-05-27. If you feel this issue or pull request deserves attention, please reopen the issue. Please see this wiki page for more information. Thank you for your contributions.

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