feat(sqlparser): allow rhs of AT TIME ZONE to be non-literal

[xiangjinwu]

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What's changed and what's your intention?

Before: t.local AT TIME ZONE 'US/Pacific'
After: t.local AT TIME ZONE t.zone

Resolves #17384

Checklist

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Documentation

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Release note

The time zone parameter of AT TIME ZONE is no longer limited to literals and can be arbitrary varchar expression.