fix: remove bartering drop order bias by shuffling candidates (#4326)

[1vers1on]

Description

This PR fixes the issue where the piglin barter drop uses a HashSet for iteration, which results in a deterministic order after the set is finalized.

Proposed changes

• Added a new method in SlimefunRegistry called getRandomizedBarteringDrops(), which copies the barterDrops into a List and shuffles it with ThreadLocalRandom.
• Updated the piglin drop event listener to use the randomized list instead of iterating over the original HashSet.

Related Issues (if applicable)

Resolves #4326

Checklist

• I have fully tested the proposed changes and promise that they will not break everything into chaos.
• I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.16.* - 1.20.*).
• I followed the existing code standards and didn't mess up the formatting.
• I did my best to add documentation to any public classes or methods I added.
• I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• I added sufficient Unit Tests to cover my code.

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[github-actions]

Slimefun preview build

A Slimefun preview build is available for testing!
Commit: 8d9f1a1

https://preview-builds.walshy.dev/download/Slimefun/4337/8d9f1a12

Note: This is not a supported build and is only here for the purposes of testing.
Do not run this on a live server and do not report bugs anywhere but this PR!