fix: Simplify enforce instance type policy by using sets

[FidelusAleksander]

Description of the change

Helper functions and iteration seem unnecessary here

Checklist

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• Each new policy has corresponding tests.
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Note

Low Risk
Small refactor of a sample Rego policy that changes data structures and membership checks but keeps the same enforcement intent.

Overview
Refactors examples/plan/enforce-instance-type-list.rego to represent allow_list/deny_list as Rego sets instead of arrays and performs membership tests via direct set indexing.

Removes the is_in_allow_list/is_in_deny_list helper rules and updates the deny/warn conditions to use the new set-based checks.

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[jubranNassar]

Hey @FidelusAleksander - Apologies for the hold up. I see @KNiepok approved but seems like there is a conflict. Can you please resolve it whenever you get the chance? you can @ me here and I'll be happy to approve and merge the changes :)