[#1650] Expose externalRef via REST API (#1655)

Signed-off-by: Marinov Avgustin <Avgustin.Marinov@bosch.com>

hawkbit-rest/hawkbit-mgmt-api/src/main/java/org/eclipse/hawkbit/mgmt/json/model/action/MgmtAction.java

@@ -116,4 +116,7 @@ public class MgmtAction extends MgmtBaseEntity {
     @Schema(description = "(Optional) Code provided as part of the last status update that was sent by the device.",
             example = "200")
     private Integer lastStatusCode;
+    @JsonProperty
+    @Schema(description = "If created by external system this field contains the external reference for the action")
+    private String externalRef;
 }