Implement react-native-flipper on iOS (#795)

Summary:
Implement the react-native-flipper native module on iOS. Uses very similar abstractions as on Android.

## Changelog

[react-native-flipper] Support iOS
Pull Request resolved: https://github.com/facebook/flipper/pull/795

Test Plan:
Tested using the RN TicTacToe example app

{F228406333}

Reviewed By: mweststrate

Differential Revision: D19853017

Pulled By: priteshrnandgaonkar

fbshipit-source-id: d93d35ff984b9ba75f812c4c8e3c82e4d550f0c0

react-native/react-native-flipper/ios/FlipperReactNativeJavaScriptPlugin.m

@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import "FlipperReactNativeJavaScriptPlugin.h"
+
+#import <FlipperKit/FlipperClient.h>
+#import <FlipperKit/FlipperResponder.h>
+
+#import "FlipperModule.h"
+
+@implementation FlipperReactNativeJavaScriptPlugin
+{
+  NSString *_pluginId;
+  BOOL _inBackground;
+}
+
+- (instancetype)initWithFlipperModule:(FlipperModule *)module
+                             pluginId:(NSString *)pluginId
+                         inBackground:(BOOL)inBackground
+{
+  if (self = [super init]) {
+    _module = module;
+    _pluginId = pluginId;
+    _inBackground = inBackground;
+  }
+  return self;
+}
+
+- (NSString *)identifier
+{
+  return _pluginId;
+}
+
+- (BOOL)runInBackground
+{
+  return _inBackground;
+}
+
+- (void)didConnect:(id<FlipperConnection>)connection
+{
+  _connection = connection;
+  [self fireOnConnect];
+}
+
+- (void)didDisconnect
+{
+  _connection = nil;
+  [_module sendEventWithName:@"react-native-flipper-plugin-disconnect"
+                        body:[self pluginParams]];
+}
+
+- (BOOL)isConnected
+{
+  return _connection != nil;
+}
+
+- (void)fireOnConnect
+{
+  [_module sendEventWithName:@"react-native-flipper-plugin-connect"
+                        body:[self pluginParams]];
+}
+
+- (id)pluginParams
+{
+  return @{@"plugin": _pluginId};
+}
+
+@end