Refactor notification system

Introduced `EnvelopeNotificationBase` as a new base class for notifications, providing a unified structure with an abstract `GetTemplateType` method and optional attachments. Added `DocumentReceivedNotification` and updated `DocumentSignedNotification` to inherit from this base class, streamlining attachment handling and template type determination.

Refactored `IEnvelopeNotificationService` to use a generic `Notify` method, allowing for flexibility with different notification types. Updated `FinalizeEnvelopeCommandHandler` to align with the new attachment handling.

Added `EnvelopeNotificationExtensions` for converting attachments to `EmailAttachmentDto` objects and introduced `ReplacePlaceholderExtensions` for dynamic placeholder replacement in email templates using reflection. Removed outdated placeholder logic from `EnvelopeNotificationService`.

EnvelopeGenerator.Application/PdfSigning/FinalizeEnvelopeCommand.cs

@@ -98,7 +98,9 @@ public class FinalizeEnvelopeCommandHandler(IRepository<Envelope> Repo, ISender
             {
                 DocumentTitle = envelope.Title,
                 NameReceiver = er.Name,
-                Attachment = doc
+                Attachments = [
+                    ($"unterzeichnetes-dokument.pdf", doc)
+                    ]
             }, cancel);
         }