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/Plugins/FlipperKitUserDefaultsPlugin/FKUserDefaultsSwizzleUtility.m

@@ -0,0 +1,76 @@
+//
+//  FKUserDefaultsSwizzleUtility.m
+//  FlipperKit
+//
+//  Created by Marc Terns on 10/6/18.
+//
+
+#import "FKUserDefaultsSwizzleUtility.h"
+#import <objc/runtime.h>
+
+@interface FKUserDefaultsSwizzleUtility ()
+@property (nonatomic, strong) NSMutableSet *swizzledClasses;
+@property (nonatomic, strong) NSMutableDictionary *swizzledBlocks;
+@property (nonatomic) IMP forwardingIMP;
+@end
+
+@implementation FKUserDefaultsSwizzleUtility
+
+- (instancetype)init {
+    if (self = [super init]) {
+        _swizzledClasses = [NSMutableSet set];
+        _swizzledBlocks = [NSMutableDictionary dictionary];
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wundeclared-selector"
+        _forwardingIMP = class_getMethodImplementation([NSObject class], @selector(flipperKitThisMethodShouldNotExist));
+#pragma clang diagnostic pop
+    }
+    return self;
+}
+
++ (instancetype)sharedInstance {
+    static FKUserDefaultsSwizzleUtility *sharedInstance = nil;
+    static dispatch_once_t onceToken = 0;
+    dispatch_once(&onceToken, ^{
+        sharedInstance = [[self alloc] init];
+    });
+    return sharedInstance;
+}
+
++ (void)swizzleSelector:(SEL)selector class:(Class)aClass block:(void (^)(NSInvocation * _Nonnull))block {
+    [[self sharedInstance] swizzleSelector:selector class:aClass block:block];
+}
+
+- (void)swizzleSelector:(SEL)selector class:(Class)aClass block:(void (^)(NSInvocation * _Nonnull))block {
+    if (![self.swizzledClasses containsObject:aClass]) {
+        SEL fwdSel = @selector(forwardInvocation:);
+        Method m = class_getInstanceMethod(aClass, fwdSel);
+        __block IMP orig;
+        __weak typeof(self) weakSelf = self;
+        IMP imp = imp_implementationWithBlock(^(id self, NSInvocation *invocation) {
+            NSString * selStr = NSStringFromSelector([invocation selector]);
+            void (^block)(NSInvocation *) = weakSelf.swizzledBlocks[aClass][selStr];
+            if (block != nil) {
+                NSString *originalStr = [@"comfacebookFlipperKit_" stringByAppendingString:selStr];
+                [invocation setSelector:NSSelectorFromString(originalStr)];
+                block(invocation);
+            } else {
+                ((void (*)(id, SEL, NSInvocation *))orig)(self, fwdSel, invocation);
+            }
+        });
+        orig = method_setImplementation(m, imp);
+        [self.swizzledClasses addObject:aClass];
+    }
+    NSMutableDictionary *classDict = self.swizzledBlocks[aClass];
+    if (classDict == nil) {
+        classDict = [NSMutableDictionary dictionary];
+        self.swizzledBlocks[(id)aClass] = classDict;
+    }
+    classDict[NSStringFromSelector(selector)] = block;
+    Method m = class_getInstanceMethod(aClass, selector);
+    NSString *newSelStr = [@"comfacebookFlipperKit_" stringByAppendingString:NSStringFromSelector(selector)];
+    SEL newSel = NSSelectorFromString(newSelStr);
+    class_addMethod(aClass, newSel, method_getImplementation(m), method_getTypeEncoding(m));
+    method_setImplementation(m, self.forwardingIMP);
+}
+@end