Added constant for selecting a USD asset for use in a Shared API/SharedSymbol

CryptoExchange.Net/SharedApis/SharedSymbol.cs

@@ -33,6 +33,11 @@ namespace CryptoExchange.Net.SharedApis
         /// </summary>
         public DateTime? DeliverTime { get; set; }
 
+        /// <summary>
+        /// Use this constant to dynamically select a USD or USD equivalent asset (stable coin) supported on the exchange
+        /// </summary>
+        public const string UsdOrStable = "CE-UsdStable";
+
         /// <summary>
         /// Create a new SharedSymbol
         /// </summary>