enhance RequestHandlingRSocket for more flexible usage as a builder for adding services (#23)

Author: OlegDokuka

rsocket-rpc-core/src/main/java/io/rsocket/rpc/rsocket/RequestHandlingRSocket.java

@@ -25,11 +25,21 @@ public RequestHandlingRSocket(RSocketRpcService... services) {
     }
   }
 
+  /**
+   * @deprecated in favour of {@link #withService(RSocketRpcService)}
+   * @param rsocketService
+   */
+  @Deprecated
   public void addService(RSocketRpcService rsocketService) {
     String service = rsocketService.getService();
     registeredServices.put(service, rsocketService);
   }
 
+  public RequestHandlingRSocket withService(RSocketRpcService rSocketRpcService) {
+    addService(rSocketRpcService);
+    return this;
+  }
+
   @Override
   public Mono<Void> fireAndForget(Payload payload) {
     try {