fix: change to generic type

This is to receive the value without know how deserialize the value

GreenArrow.Engine/Model/Events/DeliveryAttempt.cs

@@ -150,7 +150,7 @@ public enum StatusEnum
         /// Folded headers will simply contain the folding newlines/whitespace.
         /// As a header may be included in an email multiple times, the value of the JSON object is an array of strings.
         /// </summary>
-        public ICollection<string> Headers { get; init; }
+        public object Headers { get; init; }
 
         /// <summary>
         /// The size of the message (in bytes), if loaded from storage.