feat(receive): allow servers to customize vender item unpack structure

[baphomello]

Summary

This PR makes the vender_items_list item unpack process customizable for servers with modified packet structures.

Previously, the unpacked fields were mapped using a fixed key order inside Receive.pm. Some servers change the order or structure of these fields, which required editing core code.

This change introduces vender_items_list_item_keys, allowing server-specific receive modules to define the field order that corresponds to vender_items_list_item_pack.

If no custom keys are defined, the original default order is used, ensuring full backward compatibility.

Motivation

Some servers modify packet layouts for vender item lists. Allowing configurable keys avoids hardcoding structures in the core and simplifies support for custom servers.

Changes

• Added support for vender_items_list_item_keys
• Replaced fixed key assignment with configurable key list
• Preserved default behavior for existing servers