Add ReceiverAppType property and enum to SigningCommand

Added a new `ReceiverAppType` property to the `SigningCommand`
record, initialized to `ReceiverAppType.ReceiverUI`. Introduced
a `ReceiverAppType` enum with values `ReceiverUI` and `LegacyWeb`.
Updated `SignCommandHandler` to reflect these changes without
modifying its functionality.

EnvelopeGenerator.Application/DocReceiverElements/Commands/SigningCommand.cs

@@ -36,6 +36,11 @@ public record SigningCommand : EnvelopeReceiverQueryBase, IRequest
     /// 
     /// </summary>
     public IEnumerable<Signature>? Signatures { get; init; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public ReceiverAppType ReceiverAppType { get; init; } = ReceiverAppType.ReceiverUI;
 }
 
 /// <summary>
@@ -54,4 +59,20 @@ public class SignCommandHandler : IRequestHandler<SigningCommand>
     {
         return Task.CompletedTask;
     }
+}
+
+/// <summary>
+/// 
+/// </summary>
+public enum ReceiverAppType
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    ReceiverUI = 0,
+
+    /// <summary>
+    /// 
+    /// </summary>
+    LegacyWeb = 1,
 }