allow global property in values schema

[muratso]

Please provide a description of this PR:
Fix for #35495 - allows the official istio/gateway chart to be used as a subchart.
The original PR (#35496) had no interaction from the Author since October last year. So I'm opening a new one.

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[howardjohn]

thanks!

[muratso]

@howardjohn can you please re-run the cla/google job?

[istio-testing]

Hi @muratso. 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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[Arhell]

/ok-to-test