chore: declare functional interfaces effectively as functional ones

src/main/kotlin/application/service/ApplicationService.kt

@@ -13,7 +13,7 @@ package application.service
  * An Application Service handle application logic without business elements.
  * @param[T] the type returned by the service.
  */
-interface ApplicationService<out T> {
+fun interface ApplicationService<out T> {
     /**
      * Method to execute the application service.
      * @return the result of type [T]

src/main/kotlin/usecase/UseCase.kt

@@ -11,7 +11,7 @@ package usecase
 /**
  * This models a simple use case that return an object of type [X].
  */
-interface UseCase<out X> {
+fun interface UseCase<out X> {
     /** Execute the use case returning an object of type [X]. */
     fun execute(): X
 }