Add SenderAppType property to SignatureDto

Updated the `using` directives in `SignatureDto.cs` to include the `EnvelopeGenerator.Domain.Constants` namespace and reordered the existing namespaces. Added a new `SenderAppType` property to the `SignatureDto` class, with a default value of `SenderAppType.LegacyFormApp`.

EnvelopeGenerator.Application/Common/Dto/SignatureDto.cs

@@ -1,4 +1,5 @@
-using EnvelopeGenerator.Domain.Interfaces;
+using EnvelopeGenerator.Domain.Constants;
+using EnvelopeGenerator.Domain.Interfaces;
 using Microsoft.AspNetCore.Mvc;
 
 namespace EnvelopeGenerator.Application.Common.Dto;
@@ -98,4 +99,9 @@ public class SignatureDto : ISignature
     /// 
     /// </summary>
     public IEnumerable<AnnotationDto>? Annotations { get; set; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public SenderAppType SenderAppType { get; set; } = SenderAppType.LegacyFormApp;
 }