Refactor Role constants; add Receiver class, mark obsolete

Refactored the Role class by introducing a nested static Receiver class containing PreAuth and FullyAuth constants. Marked the original Role.PreAuth and Role.FullyAuth as [Obsolete] with guidance to use the new Receiver constants. Added a conditional using directive for System for NETFRAMEWORK compatibility.

EnvelopeGenerator.Domain/Constants/Role.cs

@@ -1,8 +1,21 @@
-namespace EnvelopeGenerator.Domain.Constants
+#if NETFRAMEWORK
+using System;
+#endif
+
+namespace EnvelopeGenerator.Domain.Constants
 {
     public static class Role
     {
+        [Obsolete("Use Receiver.PreAuth or Receiver.FullyAuth")]
         public const string PreAuth = "PreAuth";
+
+        [Obsolete("Use Receiver.PreAuth or Receiver.FullyAuth")]
         public const string FullyAuth = "FullyAuth";
+
+        public static class Receiver
+        {
+            public const string PreAuth = "PreAuth";
+            public const string FullyAuth = "FullyAuth";
+        }
     }
 }