fix: generate image thumbnails and use transparent image as placeholder for cached network images (#310)

packages/stream_chat_flutter/lib/src/attachment/giphy_attachment.dart

@@ -1,6 +1,9 @@
+import 'dart:typed_data';
+
 import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 import 'package:stream_chat_flutter_core/stream_chat_flutter_core.dart';
+import 'package:transparent_image/transparent_image.dart';
 
 import '../full_screen_media.dart';
 import '../stream_chat_theme.dart';
@@ -348,9 +351,7 @@ class GiphyAttachment extends AttachmentWidget {
                 return Container(
                   width: size?.width,
                   height: size?.height,
-                  child: Center(
-                    child: CircularProgressIndicator(),
-                  ),
+                  child: Image.memory(kTransparentImage),
                 );
               },
               imageUrl: imageUrl,