Add Game Service

[vadrsa]

Add a new file ./src/games/game.service.js

Implement the following methods inside the service:

• findOne(id): only the user who created the game has access to this method
• makeMove(id, i, j): only the user who created the game has access to this method, returns the updated game entity
• create(userId, difficulty): create a game for the user with the specified difficulty