Retrieve the inner SNI in GetRequestedServerName()

[ronenschafferibm]

What this PR does / why we need it:
This PR demonstrates a simple possible solution for envoyproxy/envoy#4076
by returning the inner SNI if exists instead of the outer SNI

Which issue this PR fixes
envoyproxy/envoy#4076

Special notes for your reviewer:

Release note:

[istio-testing]

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[istio-testing]

Hi @ronenschafferibm. 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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