Add IEnvelope interface and IsReadAndConfirm extension

Introduced the IEnvelope interface with an EnvelopeTypeId property in the new EnvelopeGenerator.Domain.Interfaces namespace. Added the EnvelopeExtensions static class with an IsReadAndConfirm extension method to check if EnvelopeTypeId equals 2.

EnvelopeGenerator.Domain/Interfaces/IEnvelope.cs

@@ -0,0 +1,15 @@
+namespace EnvelopeGenerator.Domain.Interfaces
+{
+    public interface IEnvelope
+    {
+        int? EnvelopeTypeId { get; set; }
+    }
+
+    public static class EnvelopeExtensions
+    {
+        public static bool IsReadAndConfirm(this IEnvelope envelope)
+        {
+            return envelope.EnvelopeTypeId == 2;
+        }
+    }
+}