SLING-5643 Provide a simple messaging API

Add type parameter for result
diff --git a/src/main/java/org/apache/sling/commons/messaging/MessageService.java b/src/main/java/org/apache/sling/commons/messaging/MessageService.java
index 05aa79e..c7d7646 100644
--- a/src/main/java/org/apache/sling/commons/messaging/MessageService.java
+++ b/src/main/java/org/apache/sling/commons/messaging/MessageService.java
@@ -24,12 +24,12 @@
 import org.osgi.annotation.versioning.ProviderType;
 
 @ProviderType
-public interface MessageService<T> {
+public interface MessageService<T, S> {
 
     /**
      * @param message the message to send
-     * @return the sent message
+     * @return the result of send operation
      */
-    CompletableFuture<T> sendMessage(@NotNull final T message);
+    @NotNull CompletableFuture<S> sendMessage(@NotNull final T message);
 
 }