let users use custom reaction builders

packages/stream_chat_flutter/lib/src/reaction_icon.dart

@@ -1,14 +1,20 @@
+import 'package:flutter/material.dart';
+
 /// Reaction icon data
 class ReactionIcon {
   /// Constructor for creating [ReactionIcon]
   ReactionIcon({
     required this.type,
-    required this.assetName,
+    required this.builder,
   });
 
   /// Type of reaction
   final String type;
 
   /// Asset to display for reaction
-  final String assetName;
+  final Widget Function(
+    BuildContext,
+    bool highlighted,
+    double size,
+  ) builder;
 }