Add Optional match fields support to PI subsystem

Change-Id: Ic458f59cab98340e40c04a0ad060d3c725ac5dbb
diff --git a/core/api/src/main/java/org/onosproject/net/pi/model/PiMatchType.java b/core/api/src/main/java/org/onosproject/net/pi/model/PiMatchType.java
index f4fce3e..a0cb161 100644
--- a/core/api/src/main/java/org/onosproject/net/pi/model/PiMatchType.java
+++ b/core/api/src/main/java/org/onosproject/net/pi/model/PiMatchType.java
@@ -42,5 +42,10 @@
     /**
      * Range match type.
      */
-    RANGE
+    RANGE,
+
+    /**
+     * Optional match type.
+     */
+    OPTIONAL
 }