[core] add extension to convert user into loggable string

Core/Extensions/UserExtension.cs

@@ -20,5 +20,12 @@ namespace Kruzya.TelegramBot.Core.Extensions
             }
             return text.Append("</a>").ToString();
         }
+
+        public static string ToLog(this User user)
+        {
+            var name = $"{user.FirstName} {user.LastName}".Trim();
+
+            return $"{name} (ID: {user.Id}, Username: {user.Username ?? "N/A"})";
+        }
     }
 }