Implement client interceptors for grpc-native-core

A NodeJS implementation of client-side interceptors, as described in
the proposal: grpc/proposal#14

Author: drobertduke

packages/grpc-native-core/index.js

@@ -200,6 +200,8 @@ exports.writeFlags = constants.writeFlags;
 
 exports.logVerbosity = constants.logVerbosity;
 
+exports.methodTypes = constants.methodTypes;
+
 exports.credentials = require('./src/credentials.js');
 
 /**
@@ -243,6 +245,11 @@ exports.getClientChannel = client.getClientChannel;
 
 exports.waitForClientReady = client.waitForClientReady;
 
+exports.StatusBuilder = client.StatusBuilder;
+exports.ListenerBuilder = client.ListenerBuilder;
+exports.RequesterBuilder = client.RequesterBuilder;
+exports.InterceptingCall = client.InterceptingCall;
+
 /**
  * @memberof grpc
  * @alias grpc.closeClient