Add SignCommand and handler for document signing

Introduce `SignCommand` and `SignCommandHandler` in the `EnvelopeGenerator.Application.Signature.Commands` namespace.

- `SignCommand` is a CQRS command that encapsulates the envelope receiver's information (`EnvelopeReceiverDto`) and optional PSPDFKit annotation data.
- Added computed properties for email template type (`TemplateType`) and receiver's email address (`EmailAddress`), with validation for null receivers.
- `SignCommandHandler` implements the `IRequestHandler<SignCommand>` interface, with a placeholder `Handle` method delegating processing to pipeline behaviors.
- Added necessary `using` directives for MediatR, DTOs, extensions, and constants.

EnvelopeGenerator.Application/Signature/Commands/SignCommand.cs

@@ -0,0 +1,52 @@
+using MediatR;
+using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.Common.Dto.EnvelopeReceiver;
+using EnvelopeGenerator.Application.Common.Extensions;
+using EnvelopeGenerator.Domain.Constants;
+
+namespace EnvelopeGenerator.Application.Signature.Commands;
+
+/// <summary>
+/// Command to sign a document by a receiver.
+/// </summary>
+public record SignCommand : IRequest
+{
+    /// <summary>
+    /// The envelope receiver information.
+    /// </summary>
+    public required EnvelopeReceiverDto EnvelopeReceiver { get; init; }
+
+    /// <summary>
+    /// The PSPDFKit annotation data.
+    /// </summary>
+    public PsPdfKitAnnotation? PsPdfKitAnnotation { get; init; }
+
+    /// <summary>
+    /// Gets the email template type.
+    /// </summary>
+    public EmailTemplateType TemplateType => EmailTemplateType.DocumentSigned;
+
+    /// <summary>
+    /// Gets the email address of the receiver.
+    /// </summary>
+    public string EmailAddress => EnvelopeReceiver.Receiver?.EmailAddress
+        ?? throw new InvalidOperationException($"Receiver is null." +
+            $"DocSignedNotification:\n{this.ToJson(Format.Json.ForDiagnostics)}");
+}
+
+/// <summary>
+/// Handles the sign command. The actual work is done by SignCommandBehavior pipeline.
+/// </summary>
+public class SignCommandHandler : IRequestHandler<SignCommand>
+{
+    /// <summary>
+    /// Executes the signing command. Pipeline behaviors handle the actual processing.
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public Task Handle(SignCommand request, CancellationToken cancellationToken = default)
+    {
+        return Task.CompletedTask;
+    }
+}