chore: add UserAvatar placeholder builder to stream_chat_theme.dart and user_avatar.dart

packages/stream_chat_flutter/lib/src/stream_chat_theme.dart

@@ -113,6 +113,7 @@ class StreamChatThemeData {
     required this.ownMessageTheme,
     required this.messageInputTheme,
     required this.defaultUserImage,
+    this.placeholderUserImage,
     required this.primaryIconTheme,
     required this.reactionIcons,
     required this.galleryHeaderTheme,
@@ -170,6 +171,9 @@ class StreamChatThemeData {
   /// The widget that will be built when the user image is unavailable
   final Widget Function(BuildContext, User) defaultUserImage;
 
+  /// The widget that will be built when the user image is loading
+  final Widget Function(BuildContext context, User user)? placeholderUserImage;
+
   /// Primary icon theme
   final IconThemeData primaryIconTheme;
 

packages/stream_chat_flutter/lib/src/user_avatar.dart

@@ -79,6 +79,8 @@ class UserAvatar extends StatelessWidget {
                   imageUrl: user.extraData['image'] as String,
                   errorWidget: (_, __, ___) =>
                       streamChatTheme.defaultUserImage(context, user),
+                  placeholder: (_, __) =>
+                      streamChatTheme.placeholderUserImage!(context, user),
                   fit: BoxFit.cover,
                 )
               : streamChatTheme.defaultUserImage(context, user),