Add truncate instruction and support it with PI framework

To support truncate by P4Runtime clone/mirror session, we need to pass the
truncate size/length from ONOS northbound to the southbound.
As discussed in the SDFabric syncup, we decide to pass this information via
the instruction in group bucket so applications or pipeliners can simply
reuse current APIs.

Change-Id: I15cc822b7c8008b6b9f8b02f3f399769ae396ef0
(cherry picked from commit 9f94a13bf5695996708eedc17166b5b09308147f)
diff --git a/core/api/src/main/java/org/onosproject/net/flow/instructions/Instruction.java b/core/api/src/main/java/org/onosproject/net/flow/instructions/Instruction.java
index 66fbb18..a87f3a3 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/instructions/Instruction.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/instructions/Instruction.java
@@ -101,7 +101,12 @@
         /**
          * Signifies that statistics will be triggered.
          */
-        STAT_TRIGGER
+        STAT_TRIGGER,
+
+        /**
+         * Signifies that the packet should be truncated.
+         */
+        TRUNCATE
     }
 
     /**