Replace the hard-coded RabbitMQ setup with NotificationOptions bound from a configurable section. The new ConsumerType switch wires either the HTTP API client or the RabbitMQ client, and startup now fails with a clear error when required settings are missing.
Bump DigitalData.MessagingService.Client to 3.0.1 for the abstraction used by both transports.
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 Removed outdated placeholder logic from `EnvelopeNotificationService`.
