fix: item purchasing spawn exploit

[Qwerty1Verified]

Description

This PR fixes an exploit that allows a client to spawn items without the appropriate checks and validation.
To perform this validation, it uses the data given by qb-shops handle checks like checking if the slot given by the client is the same as the slot given by the shop, and rather than using the price given by the client, instead use the price given by the shop.

Checklist

• I have personally loaded this code into an updated qbcore project and checked all of its functionality.
• My code fits the style guidelines.
• My PR fits the contribution guidelines.