Add custom exception classes and Extensions folder

Introduce four custom exceptions (BurnAnnotationException, CreateReportException, ExportDocumentException, MergeDocumentException) under EnvelopeGenerator.ServiceHost.Exceptions for improved error handling. Update the project file to include the new Extensions folder.

EnvelopeGenerator.ServiceHost/Exceptions/BurnAnnotationException.cs

@@ -0,0 +1,12 @@
+namespace EnvelopeGenerator.ServiceHost.Exceptions;
+
+public class BurnAnnotationException : ApplicationException
+{
+    public BurnAnnotationException(string message) : base(message)
+    {
+    }
+
+    public BurnAnnotationException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}