[Feature Request]: Add an option to set slots for preview items

[ryderbelserion]

Discussed in https://github.com/Crazy-Crew/CrazyCrates/discussions/569

^{Originally posted by MassiveLag June 30, 2023}

Describe the feature

Hi, add under reward items an option like

previewSlot:

This will make it so that you can make custom slots in preview gui.