refactor: convert BurnPdfCommand to record and update handler to return byte[]

- Changed `BurnPdfCommand` from empty class to a `record` with properties: `SourceBuffer`, `InstantJSONList`, `EnvelopeId`.
- Updated `BurnPdfCommandHandler` to implement `IRequestHandler<BurnPdfCommand, byte[]>`.
- Modified `Handle` method to be async and use the `request` data to return PDF bytes.
- Removed obsolete `BurnPdfCommand` class and updated related logic accordingly.

EnvelopeGenerator.Domain/Entities/Signature.cs

@@ -2,10 +2,11 @@
 using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
-using System.Text.Json.Serialization;
 #if NETFRAMEWORK
 using System;
 using System.Collections.Generic;
+#elif NET
+using System.Text.Json.Serialization;
 #endif
 
 namespace EnvelopeGenerator.Domain.Entities