Changed the `Attachments` property type in `EnvelopeNotificationBase.cs` from `List` to `IReadOnlyList` to enhance immutability and thread safety. Updated the `FinalizeEnvelopeCommandHandler` in `FinalizeEnvelopeCommand.cs` to use a single-line syntax for `Attachments` initialization, improving code readability.
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 Removed outdated placeholder logic from `EnvelopeNotificationService`.
