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/OutgoingEmailEvent.cs

@@ -0,0 +1,30 @@
+using System;
+
+namespace DigitalData.MessagingService.Publisher.Abstraction;
+
+public class OutgoingEmailEvent
+{
+    public Guid Id { get; set; }
+
+    /// <summary>
+    /// Recipient email address
+    /// </summary>
+    public string Recipient { get; set; } = null!;
+
+    /// <summary>
+    /// Email subject
+    /// </summary>
+    public string Subject { get; set; } = null!;
+
+    /// <summary>
+    /// Email body (HTML or plain text)
+    /// </summary>
+    public string Body { get; set; } = null!;
+
+    /// <summary>
+    /// Is HTML email (default: true)
+    /// </summary>
+    public bool IsHtml { get; set; } = true;
+
+    public DateTime QueuedAt { get; set; }
+}