Add support for ephemeral middlewares.

src/main/java/pw/mihou/nexus/features/command/facade/NexusMiddlewareEvent.java

@@ -14,13 +14,54 @@ public interface NexusMiddlewareEvent extends NexusCommandEvent {
      * A middleware-only function that tells Discord that the response will be
      * taking more than three-seconds because the middleware has to process tasks
      * that can take more than three-second limit.
+     * @return The future to determine whether the response was accepted or not.
      */
     default CompletableFuture<Void> askDelayedResponse() {
             return CompletableFuture.allOf(
                     getNexus().getResponderRepository().peek(getBaseEvent().getInteraction())
             );
     }
 
+    /**
+     * A middleware-only function that tells Discord that the response will be
+     * taking more than three-seconds because the middleware has to process tasks
+     * that can take more than three-second limit.
+     * @return The future to determine whether the response was accepted or not.
+     */
+    default CompletableFuture<Void> askDelayedResponseAsEphemeral() {
+        return CompletableFuture.allOf(
+                getNexus().getResponderRepository().peekEphemeral(getBaseEvent().getInteraction())
+        );
+    }
+
+    /**
+     * A middleware-only function that tells Discord that the response will be
+     * taking more than three-seconds because the middleware has to process tasks
+     * that can take more than three-second limit.
+     *
+     * @param predicate The predicate to determine whether the response should be ephemeral or not.
+     * @return The future to determine whether the response was accepted or not.
+     */
+    default CompletableFuture<Void> askDelayedResponseAsEphemeralIf(boolean predicate) {
+        if (predicate) {
+            return askDelayedResponseAsEphemeral();
+        }
+
+        return askDelayedResponse();
+    }
+
+    /**
+     * A middleware-only function that tells Discord that the response will be
+     * taking more than three-seconds because the middleware has to process tasks
+     * that can take more than three-second limit.
+     *
+     * @param predicate The predicate to determine whether the response should be ephemeral or not.
+     * @return The future to determine whether the response was accepted or not.
+     */
+    default CompletableFuture<Void> askDelayedResponseAsEphemeralIf(Predicate<Void> predicate) {
+        return askDelayedResponseAsEphemeralIf(predicate.test(null));
+    }
+
     /**
      * Tells the command interceptor handler to move forward with the next
      * middleware if there is any, otherwise executes the command code.

src/main/java/pw/mihou/nexus/features/command/responders/NexusResponderRepository.java

@@ -35,6 +35,30 @@ public CompletableFuture<InteractionOriginalResponseUpdater> peek(Interaction in
                 });
     }
 
+    /**
+     * Gets the current {@link InteractionOriginalResponseUpdater} for the specific interaction
+     * if available from another middleware otherwise requests for a new {@link InteractionOriginalResponseUpdater}
+     * that can be used instead.
+     * <br><br>
+     * Not to be confused with {@link NexusResponderRepository#get(Interaction)} which deliberately
+     * destroys the interaction that is being stored after being requested. This is intended for middlewares to
+     * prevent Discord's Interaction has failed while processing a heavy task.
+     *
+     * @param interaction   The interaction to reference.
+     * @return              The {@link InteractionOriginalResponseUpdater} if present otherwise requests for one.
+     */
+    public CompletableFuture<InteractionOriginalResponseUpdater> peekEphemeral(Interaction interaction) {
+        if (responders.containsKey(interaction.getId())) {
+            return CompletableFuture.completedFuture(responders.get(interaction.getId()));
+        }
+
+        return interaction.respondLater(true)
+                .thenApply(responseUpdater -> {
+                    responders.put(interaction.getId(), responseUpdater);
+                    return responseUpdater;
+                });
+    }
+
     /**
      * Gets the current {@link InteractionOriginalResponseUpdater} for the specific interaction if
      * available from another middleware otherwise requests for a new {@link InteractionOriginalResponseUpdater}