Refactor Layered Architecture pattern

[iluwatar]

A couple of specific refactorings are needed for Layered Architecture code example.

• Runner.java has no main entry point and can't be executed in the IDE. Let's add this.
• 3 Warnings: Optional used as type for field id. This is related to classes CakeInfo, CakeLayerInfo, and CakeToppingInfo.

Optional is primarily intended for use as a method return type where there is a clear need to represent "no result," and where using null is likely to cause errors.

Acceptance criteria

• Refactorings implemented

[romannimets]

Hey! I would like to contribute to this issue