Add metadata.customAttributes field

Summary:
`customAttributes` will be used by the UI Debugger to pass styleID for Bloks model attributes.
(replicating corresponding [Android implementation](https://www.internalfb.com/code/fbsource/[33c33fa7f582]/xplat/sonar/android/src/facebook/java/com/facebook/flipper/uidebugger/bloks/descriptors/BloksDebugComponentDescriptor.kt?lines=138-139%2C170-171))

Reviewed By: antonk52

Differential Revision: D47494013

fbshipit-source-id: 257387d4af94235b23d636047326532d7071e8dd

iOS/Plugins/FlipperKitUIDebuggerPlugin/FlipperKitUIDebuggerPlugin/Serializers/UIDMetadata+Foundation.m

@@ -7,6 +7,7 @@
 
 #if FB_SONARKIT_ENABLED
 
+#import "NSDictionary+Foundation.h"
 #import "NSSet+Foundation.h"
 #import "UIDMetadata+Foundation.h"
 
@@ -14,12 +15,20 @@ FB_LINKABLE(UIDMetadata_Foundation)
 @implementation UIDMetadata (Foundation)
 
 - (id)toFoundation {
-  return @{
+  NSMutableDictionary* result = [NSMutableDictionary dictionary];
+
+  [result addEntriesFromDictionary:@{
     @"id" : self.identifier,
     @"type" : self.type,
     @"name" : self.name,
     @"mutable" : [NSNumber numberWithBool:self.isMutable],
-  };
+  }];
+
+  if (self.customAttributes != nil) {
+    result[@"customAttributes"] = [self.customAttributes toFoundation];
+  }
+
+  return [result copy];
 }
 
 @end