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

- implement to entity and dto

EnvelopeGenerator.Application/Common/Dto/SignatureDto.cs

@@ -1,4 +1,5 @@
-using Microsoft.AspNetCore.Mvc;
+using EnvelopeGenerator.Domain.Interfaces;
+using Microsoft.AspNetCore.Mvc;
 
 namespace EnvelopeGenerator.Application.Common.Dto;
 
@@ -6,7 +7,7 @@ namespace EnvelopeGenerator.Application.Common.Dto;
 /// Data Transfer Object representing a positioned element assigned to a document receiver.
 /// </summary>
 [ApiExplorerSettings(IgnoreApi = true)]
-public class SignatureDto
+public class SignatureDto : ISignature
 {
     /// <summary>
     /// Gets or sets the unique identifier of the element.