feat(Format): create to centrilize the formats.

- Add json serializer settings for diagnostics and implement to DocumentController

EnvelopeGenerator.Domain/Constants/Format.cs

@@ -0,0 +1,20 @@
+using Newtonsoft.Json;
+using Newtonsoft.Json.Serialization;
+
+namespace EnvelopeGenerator.Domain.Constants
+{
+    public class Format
+    {
+        #region Json Serializer Settings
+        public static class Json
+        {
+            public static readonly JsonSerializerSettings ForDiagnostics = new JsonSerializerSettings()
+            {
+                ContractResolver = new CamelCasePropertyNamesContractResolver(),
+                Formatting = Formatting.Indented,
+                NullValueHandling = NullValueHandling.Include
+            };
+        }
+        #endregion
+    }
+}