feat(Core/Scripting): Implement hook OnAfterCalculateLootGroupAmount()

[hallgaeuer]

Changes Proposed:

This PR proposes changes to:

• Core (units, players, creatures, game systems).
• Scripts (bosses, spell scripts, creature scripts).
• Database (SAI, creatures, etc).

Adds a hook OnAfterCalculateLootGroupAmount to the loot manager directly after the config option Rate.Drop.Item.GroupAmount is read. This hook allows modules to dynamically modify the drop amount of grouped loot items.

A similar hook already exists for reference loot (OnAfterRefCount).

I'm not to sure about the name OnAfterCalculateLootGroupAmount. If somebody has a better name I'm open to it.

Issues Addressed:

SOURCE:

The changes have been validated through:

• Live research (checked on live servers, e.g Classic WotLK, Retail, etc.)
• Sniffs (remember to share them with the open source community!)
• Video evidence, knowledge databases or other public sources (e.g forums, Wowhead, etc.)
• The changes promoted by this pull request come partially or entirely from another project (cherry-pick). Cherry-picks must be committed using the proper --author tag in order to be accepted, thus crediting the original authors, unless otherwise unable to be found

Tests Performed:

This PR has been:

• Tested in-game by the author.
• Tested in-game by other community members/someone else other than the author/has been live on production servers.
• This pull request requires further testing and may have edge cases to be tested.

How to Test the Changes:

• This pull request can be tested by following the reproduction steps provided in the linked issue
• This pull request requires further testing. Provide steps to test your changes. If it requires any specific setup e.g multiple players please specify it as well.

1. Create a simple module that implements the hook
2. Modify the parameter "groupAmount" passed to a higher value
3. Observe higher drop amounts of grouped loot items

I have implemented a module to allow custom drop amounts in dungeons / raids that use this hook: https://github.com/hallgaeuer/mod-dynamic-loot-rates
The hook seems to work fine in my personal tests.

Known Issues and TODO List:

None

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[kjack9]

I LOVE this. I have in my backlog to add a "Loot Chance Modifier" to AutoBalance, and this hook would make that super easy.