Add EnvelopeSigningType enum and Normalize extension

Introduced EnvelopeSigningType enum with WetSignature and ReadAndSign values in the EnvelopeGenerator.Domain.Constants namespace. Added EnvelopeSigningTypeExtensions with a Normalize method to standardize enum values.

EnvelopeGenerator.Domain/Constants/EnvelopeSigningType.cs

@@ -0,0 +1,14 @@
+namespace EnvelopeGenerator.Domain.Constants
+{
+    public enum EnvelopeSigningType
+    {
+        WetSignature = 1,
+        ReadAndSign = 2
+    }
+
+    public static class EnvelopeSigningTypeExtensions
+    {
+        public static EnvelopeSigningType Normalize(this EnvelopeSigningType value) =>
+            value == EnvelopeSigningType.ReadAndSign ? EnvelopeSigningType.ReadAndSign : EnvelopeSigningType.WetSignature;
+    }
+}