feat(iOS): Add prop for allowingReadAccessToURL (#771)

docs/Reference.md

 - [`directionalLockEnabled`](Reference.md#directionalLockEnabled)
 - [`geolocationEnabled`](Reference.md#geolocationenabled)
 - [`allowUniversalAccessFromFileURLs`](Reference.md#allowUniversalAccessFromFileURLs)
+- [`allowingReadAccessToURL`](Reference.md#allowingReadAccessToURL)
 - [`useWebKit`](Reference.md#usewebkit)
 - [`url`](Reference.md#url)
 - [`html`](Reference.md#html)
 
 ---
 
+### `allowingReadAccessToURL`
+
+A String value that indicates which URLs the WebView's file can then reference in scripts, AJAX requests, and CSS imports. This is only used in `RNCWKWebView` for WebViews that are loaded with a source.uri set to a `'file://'` URL. If not provided, the default is to only allow read access to the URL provided in source.uri itself.
+
+| Type   | Required | Platform      |
+| ------ | -------- | ------------- |
+| string | No       | iOS WKWebView |
+
+---
+
 ### `useWebKit`
 
 If true, use WKWebView instead of UIWebView.

ios/RNCWKWebView.h

 @property (nonatomic, assign) BOOL showsHorizontalScrollIndicator;
 @property (nonatomic, assign) BOOL showsVerticalScrollIndicator;
 @property (nonatomic, assign) BOOL directionalLockEnabled;
+@property (nonatomic, copy) NSString *allowingReadAccessToURL;
 
 + (void)setClientAuthenticationCredential:(nullable NSURLCredential*)credential;
 - (void)postMessage:(NSString *)message;

ios/RNCWKWebView.m

   }
 }
 
+- (void)setAllowingReadAccessToURL:(NSString *)allowingReadAccessToURL
+{
+  if (![_allowingReadAccessToURL isEqualToString:allowingReadAccessToURL]) {
+    _allowingReadAccessToURL = [allowingReadAccessToURL copy];
+
+    if (_webView != nil) {
+      [self visitSource];
+    }
+  }
+}
+
 - (void)setContentInset:(UIEdgeInsets)contentInset
 {
   _contentInset = contentInset;
         [_webView loadRequest:request];
     }
     else {
-        [_webView loadFileURL:request.URL allowingReadAccessToURL:request.URL];
+        NSURL* readAccessUrl = _allowingReadAccessToURL ? [NSURL URLWithString:_allowingReadAccessToURL] : request.URL;
+        [_webView loadFileURL:request.URL allowingReadAccessToURL:readAccessUrl];
     }
 }
 

ios/RNCWKWebViewManager.m

 RCT_EXPORT_VIEW_PROPERTY(applicationNameForUserAgent, NSString)
 RCT_EXPORT_VIEW_PROPERTY(cacheEnabled, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(allowsLinkPreview, BOOL)
+RCT_EXPORT_VIEW_PROPERTY(allowingReadAccessToURL, NSString)
 
 #if defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && __IPHONE_OS_VERSION_MAX_ALLOWED >= 110000 /* __IPHONE_11_0 */
 RCT_EXPORT_VIEW_PROPERTY(contentInsetAdjustmentBehavior, UIScrollViewContentInsetAdjustmentBehavior)

src/WebViewTypes.ts

 }
 
 export interface IOSNativeWebViewProps extends CommonNativeWebViewProps {
+  allowingReadAccessToURL?: string;
   allowsBackForwardNavigationGestures?: boolean;
   allowsInlineMediaPlayback?: boolean;
   allowsLinkPreview?: boolean;
    * @platform ios
    */
   keyboardDisplayRequiresUserAction?: boolean;
+
+  /**
+   * A String value that indicates which URLs the WebView's file can then
+   * reference in scripts, AJAX requests, and CSS imports. This is only used
+   * in `RNCWKWebView` for WebViews that are loaded with a source.uri set to a
+   * `'file://'` URL.
+   * 
+   * If not provided, the default is to only allow read access to the URL
+   * provided in source.uri itself.
+   * @platform ios
+   */
+  allowingReadAccessToURL?: string;
 }
 
 export interface AndroidWebViewProps extends WebViewSharedProps {