Target attributes update can be (re-)triggered by management API (#690)

* extend Management API with attribute `requestAttributes`
* introduce new DMF Event Topic `REQUEST_ATTRIBUTES_UPDATE`
* enhance REST documentation by new functionality
* add tests for:
  * Management API
  * Amqp Message Dispatcher Service
  * Repository (Target Management)

Signed-off-by: Jeroen Jan Laverman (INST/ESY3) <jeroen.laverman@bosch-si.com>

hawkbit-dmf/hawkbit-dmf-api/src/main/java/org/eclipse/hawkbit/dmf/amqp/api/EventTopic.java

@@ -32,6 +32,10 @@ public enum EventTopic {
     /**
      * Topic when sending a download only task, skipping the install.
      */
-    DOWNLOAD;
+    DOWNLOAD,
+    /**
+     * Topic when an update of device attributes is requested.
+     */
+    REQUEST_ATTRIBUTES_UPDATE;
 
 }