Add Sender constant to Role in Domain.Constants

Added a new Sender constant to the Role class within the EnvelopeGenerator.Domain.Constants namespace, allowing it to be used alongside existing Receiver role constants.

EnvelopeGenerator.Domain/Constants/Role.cs

@@ -17,5 +17,7 @@ namespace EnvelopeGenerator.Domain.Constants
             public const string PreAuth = "PreAuth";
             public const string FullyAuth = "FullyAuth";
         }
+
+        public const string Sender = "Sender";
     }
 }