Refactor email template handling and error management

- Updated EmailTemplateDto to allow mutable Body and Subject properties.
- Implemented IRequest interface in UpdateEmailTemplateCommand for MediatR.
- Enhanced error handling in EmailTemplateController with NotFoundException.
- Introduced UpdateEmailTemplateCommandHandler for processing update commands.
- Added NotFoundException class for improved error handling.

EnvelopeGenerator.Application/Exceptions/NotFoundException.cs

@@ -0,0 +1,12 @@
+namespace EnvelopeGenerator.Application.Exceptions;
+
+public class NotFoundException : Exception
+{
+    public NotFoundException()
+    {
+    }
+
+    public NotFoundException(string? message) : base(message)
+    {
+    }
+}