GameAd & Order models/DTOs constraints annotations

[Vurpas]

Add constraints annotations in createGameDTO, gameAd model, order model and createOrderDTO to add restrictions to the required fields

• [ ] gameAd model constraints
• [ ] createGameAdDTO constraints
• [ ] order model constraints
• [ ] createOrderDTO constraints
• [ ] Make sure that all the methods that need a user acctually checks for an existing user in database, or throw error
• [ ] Postman Testing