Implement stage as generic to return either tables dto or brackets dto

[LuminiteTime]

Add Stageable interface, implement it with Tables(DTO), Brackets(DTO), and FinalableStage extends Stageable. TournamentDTO has field of type List<T extends Stageable>.
FinalableStage has List<Finalable> : finals == List<Finalable>.
Class Tables implements Finalable.
Class Brackets implements Finalable.
Interface Finalable extends Stageable.
Class FinalableStage implements Stageable.