Minor changes to PI runtime classes

- New class for action runtime parameter and its identifier
- PiAction builder
- Various indentifier builders

Change-Id: I265f71c868c21dbbbe633622b0c4330712f5a5ad
diff --git a/core/api/src/main/java/org/onosproject/net/pi/runtime/PiTableEntry.java b/core/api/src/main/java/org/onosproject/net/pi/runtime/PiTableEntry.java
index 741fc04..c6f6094 100644
--- a/core/api/src/main/java/org/onosproject/net/pi/runtime/PiTableEntry.java
+++ b/core/api/src/main/java/org/onosproject/net/pi/runtime/PiTableEntry.java
@@ -143,6 +143,11 @@
                 .toString();
     }
 
+    /**
+     * Returns a table entry builder.
+     *
+     * @return a new builder
+     */
     public static Builder builder() {
         return new Builder();
     }