[ReactNative] Add RCTBridgeDelegate

Summary:
Add a new bridge delegate protocol to allow a more flexible bridge configuration.

For now it just support the pre-existent configurations + providing the JavaScript
source to the bridge, that should allow pre-loading sources.

Libraries/WebSocket/RCTWebSocketExecutor.m

@@ -149,7 +149,11 @@ RCT_EXPORT_MODULE()
 
 - (void)executeApplicationScript:(NSString *)script sourceURL:(NSURL *)URL onComplete:(RCTJavaScriptCompleteBlock)onComplete
 {
-  NSDictionary *message = @{@"method": @"executeApplicationScript", @"url": [URL absoluteString], @"inject": _injectedObjects};
+  NSDictionary *message = @{
+    @"method": @"executeApplicationScript",
+    @"url": RCTNullIfNil([URL absoluteString]),
+    @"inject": _injectedObjects,
+  };
   [self sendMessage:message waitForReply:^(NSError *error, NSDictionary *reply) {
     onComplete(error);
   }];