Refactor solution structure and add RabbitMQ config

Reorganized the solution structure to align with a layered architecture:
- Replaced `src` folder with `core`, `infrastructure`, and `presentation`.
- Moved projects to their respective folders.
- Added `DigitalData.MessagingService.Publisher.Abstraction` project.
- Removed `DigitalData.MessagingService.Client` project.

Updated project configurations and nesting in the solution file.

Added `appsettings.Secrets.json` with RabbitMQ and email account settings:
- RabbitMQ configuration includes hostname, port, credentials, and queue/exchange details.
- Email configuration includes SMTP server details and credentials.

src/core/DigitalData.MessagingService.Publisher.Abstraction/IOutgoingEmailPublisher.cs

@@ -0,0 +1,14 @@
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace DigitalData.MessagingService.Publisher.Abstraction;
+
+/// <summary>
+/// Email queue interface for outgoing emails.
+/// </summary>
+public interface IOutgoingEmailPublisher
+{
+    Task EnqueueAsync(OutgoingEmailEvent outgoingEmailEvent, CancellationToken cancellationToken = default);
+
+    Task<int> GetQueueDepthAsync(CancellationToken cancellationToken = default);
+}