refactor(ISignature): create to handle common properties of a signature

- implement to entity and dto

EnvelopeGenerator.Domain/Interfaces/ISignature.cs

@@ -0,0 +1,11 @@
+namespace EnvelopeGenerator.Domain.Interfaces
+{
+    public interface ISignature
+    {
+        int Page { get; set; }
+
+        double X { get; set; }
+
+        double Y { get; set; }
+    }
+}