Add SaveSignatureBehavior and resolve merge conflicts

Introduced the `SaveSignatureBehavior` class as a pipeline behavior for handling `SigningCommand` requests. Added necessary `using` directives to include required namespaces. Resolved merge conflicts in `using` directives between `net8.0` and `net9.0` project versions. Implemented the `Handle` method to delegate request processing.

EnvelopeGenerator.Application/DocReceiverElements/Behaviors/SaveSignatureBehavior.cs

@@ -0,0 +1,51 @@
+using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.DocStatus.Commands;
+using EnvelopeGenerator.Domain.Constants;
+using MediatR;
+using System.Text.Json;
+
+<<<<<<< TODO: Unmerged change from project 'EnvelopeGenerator.Application (net8.0)', Before:
+=======
+using EnvelopeGenerator.Application.DocReceiverElements.Commands.SigningCommand.SigningCommand;
+using EnvelopeGenerator.Application.DocReceiverElements.Commands.SigningCommand;
+>>>>>>> After
+
+<<<<<<< TODO: Unmerged change from project 'EnvelopeGenerator.Application (net9.0)', Before:
+=======
+using EnvelopeGenerator.Application.DocReceiverElements.Commands.SigningCommand;
+>>>>>>> After
+using EnvelopeGenerator.Application.DocReceiverElements.Commands.SigningCommand.SigningCommand.SigningCommand;
+using EnvelopeGenerator.Application.DocReceiverElements.Commands.SigningCommand.SigningCommand;
+using EnvelopeGenerator.Application.DocReceiverElements.Commands.SigningCommand;
+
+namespace EnvelopeGenerator.Application.DocReceiverElements.Behaviors;
+
+/// <summary>
+/// Pipeline behavior that creates document status.
+/// Executes second in the signing process.
+/// </summary>
+public class SaveSignatureBehavior : IPipelineBehavior<SigningCommand, Unit>
+{
+    private readonly ISender _sender;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="sender"></param>
+    public SaveSignatureBehavior(ISender sender)
+    {
+        _sender = sender;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="next"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public async Task<Unit> Handle(SigningCommand request, RequestHandlerDelegate<Unit> next, CancellationToken cancellationToken)
+    {
+        return await next(cancellationToken);
+    }
+}