HADOOP-17819. Add extensions to ProtobufRpcEngine RequestHeaderProto. Contributed by Hector Sandoval Chaverri. (#3242)

(cherry picked from commit 3c8a48e681babe4038e49205624e7be183b3466e)

hadoop-common-project/hadoop-common/src/main/proto/ProtobufRpcEngine.proto

@@ -64,4 +64,7 @@ message RequestHeaderProto {
   
   /** protocol version of class declaring the called method */
   required uint64 clientProtocolVersion = 3;
+
+  /** protocol extensions */
+  extensions 1000 to max;
 }