refactor(llc, ui): migrate enums to v3

Signed-off-by: xsahil03x <[email protected]>

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

@@ -7,14 +7,10 @@ enum AttachmentSource {
   local,
 
   /// Attachment is uploaded
-  network,
-}
+  network;
 
-/// Extension for identifying type of attachment
-extension AttachmentSourceX on AttachmentSource {
   /// The [when] method is the equivalent to pattern matching.
   /// Its prototype depends on the AttachmentSource defined.
-  // ignore: missing_return
   T when<T>({
     required T Function() local,
     required T Function() network,