feat(wkwebview): create api to allow clients to present a client credential for authentication (#141)

* In order for TLS Mutual Auth to work for webviews, the caller must present a credential. Expose a setter that can be called to set a credential.

docs/Custom-iOS.md

 }
 ```
 
+### Setting Client Certificate Authentication Credential
+
+If you open webpages that needs a Client Certificate for Authentication, you can create a credential and pass it to the webview:
+
+```
+[RNCWKWebView setClientAuthenticationCredential:credential];
+```
+
+This can be paired with a call from Javascript to pass a string label for the certificate stored in keychain and use native calls to fetch the certificate to create a credential object. This call can be made anywhere that makes sense for your application (e.g. as part of the user authentication stack). The only requirement is to make this call before displaying any webviews.
+
 ## JavaScript Interface
 
 To use your custom web view, you'll need to create a class for it. Your class must:

ios/RNCWKWebView.h

 @property (nonatomic, assign) BOOL cacheEnabled;
 @property (nonatomic, assign) BOOL allowsLinkPreview;
 
++ (void)setClientAuthenticationCredential:(nullable NSURLCredential*)credential;
 - (void)postMessage:(NSString *)message;
 - (void)injectJavaScript:(NSString *)script;
 - (void)goForward;

ios/RNCWKWebView.m

 
 static NSTimer *keyboardTimer;
 static NSString *const MessageHandlerName = @"ReactNativeWebView";
+static NSURLCredential* clientAuthenticationCredential;
 
 // runtime trick to remove WKWebView keyboard default toolbar
 // see: http://stackoverflow.com/questions/19033292/ios-7-uiwebview-keyboard-issue/19042279#19042279
   return [[NSMutableDictionary alloc] initWithDictionary: event];
 }
 
++ (void)setClientAuthenticationCredential:(nullable NSURLCredential*)credential {
+  clientAuthenticationCredential = credential;
+}
+
+- (void)                    webView:(WKWebView *)webView
+  didReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge
+                  completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential * _Nullable))completionHandler
+{
+  if (!clientAuthenticationCredential) {
+    completionHandler(NSURLSessionAuthChallengePerformDefaultHandling, nil);
+    return;
+  }
+  if ([[challenge protectionSpace] authenticationMethod] == NSURLAuthenticationMethodClientCertificate) {
+    completionHandler(NSURLSessionAuthChallengeUseCredential, clientAuthenticationCredential);
+  } else {
+    completionHandler(NSURLSessionAuthChallengePerformDefaultHandling, nil);
+  }
+}
+
 #pragma mark - WKNavigationDelegate methods
 
 /**