Add events to DHCP L2 relay application.

The app will send events regarding the DHCP state machine.

Change-Id: I0a20d74e1a1f3b5359509611e9cf1f59495153d5
diff --git a/src/main/java/org/opencord/dhcpl2relay/DhcpL2RelayService.java b/src/main/java/org/opencord/dhcpl2relay/DhcpL2RelayService.java
new file mode 100644
index 0000000..76957ab
--- /dev/null
+++ b/src/main/java/org/opencord/dhcpl2relay/DhcpL2RelayService.java
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2018-present Open Networking Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.opencord.dhcpl2relay;
+
+import org.onosproject.event.ListenerService;
+
+/**
+ * DHCP L2 relay service.
+ */
+public interface DhcpL2RelayService extends
+        ListenerService<DhcpL2RelayEvent, DhcpL2RelayListener> {
+}