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/infrastructure/DigitalData.MessagingService.RabbitMQ/DependencyInjection.cs

@@ -0,0 +1,28 @@
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace DigitalData.MessagingService.RabbitMQ
+{
+    /// <summary>
+    /// Dependency injection configuration for Infrastructure layer
+    /// </summary>
+    public static class DependencyInjection
+    {
+        /// <summary>
+        /// Adds Infrastructure layer services to the DI container
+        /// </summary>
+        public static IServiceCollection AddRabbitMqConnectionFactory(
+            this IServiceCollection services,
+            IConfiguration configuration)
+        {
+            services.AddSingleton<RabbitMqConnectionFactory>();
+
+            // --- RabbitMQ Configuration ---
+            services.Configure<RabbitMqConfiguration>(
+                configuration.GetSection(RabbitMqConfiguration.SectionName));
+
+            return services;
+        }
+    }
+
+}