Integrating NSUserDefaults plugin to iOS (#291)

Summary:
I have a few details left, but its almost done. This PR addresses #145

- The NSUserDefaults plugin uses the SharedPreferences Desktop Part since we can reuse all of it.
- The NSUserDefaults plugin uses swizzling in order to be notified of what specific event changed at runtime.
- Added Test harness in both Sample Swift and Sample apps for iOS in order to test the plugin.
- Updated the documentation in `docs/shared-preferences-plugin.md` and` README.md`

I am open to suggestions since the desktop sharedPreferences version doesn't support deletion of preferences. Most likely I would have to modify the UI, and for that matter, I might as well build a user defaults desktop version

I wanted to add xiphirx in this MR since he developed the shared preferences plugin for Android and Desktop. I don't see a way to remove preferences from the flipper desktop app so I was wondering if you would be OK with me adding that.
Pull Request resolved: https://github.com/facebook/flipper/pull/291

Reviewed By: passy

Differential Revision: D10334685

Pulled By: priteshrnandgaonkar

fbshipit-source-id: d798c01a46df7ddecf713924799f046b560ea922

iOS/Sample/AppDelegate.mm

@@ -10,8 +10,10 @@
 #import <FlipperKit/FlipperClient.h>
 #import <FlipperKitLayoutPlugin/FlipperKitLayoutPlugin.h>
 #import <FlipperKitNetworkPlugin/FlipperKitNetworkPlugin.h>
+#import <FlipperKitUserDefaultsPlugin/FKUserDefaultsPlugin.h>
 #import <FlipperKitLayoutComponentKitSupport/FlipperKitLayoutComponentKitSupport.h>
 #import <SKIOSNetworkPlugin/SKIOSNetworkAdapter.h>
+#import <FlipperKitUserDefaultsPlugin/FKUserDefaultsPlugin.h>
 
 #import "MainViewController.h"
 #import "RootViewController.h"
@@ -27,13 +29,14 @@
 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
 {
   _window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
-
   FlipperClient *client = [FlipperClient sharedClient];
 
   SKDescriptorMapper *layoutDescriptorMapper = [[SKDescriptorMapper alloc] initWithDefaults];
   [FlipperKitLayoutComponentKitSupport setUpWithDescriptorMapper: layoutDescriptorMapper];
   [client addPlugin: [[FlipperKitLayoutPlugin alloc] initWithRootNode: application
                                                withDescriptorMapper: layoutDescriptorMapper]];
+    
+  [client addPlugin:[[FKUserDefaultsPlugin alloc] initWithSuiteName:nil]];
 
   [[FlipperClient sharedClient] addPlugin: [[FlipperKitNetworkPlugin alloc] initWithNetworkAdapter:[SKIOSNetworkAdapter new]]];
   [client start];