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.Infrastructure/DependencyInjection.cs

@@ -0,0 +1,52 @@
+using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.Infrastructure.Queue;
+using DigitalData.MessagingService.Infrastructure.Services;
+using DigitalData.MessagingService.Infrastructure.Services.Background;
+using DigitalData.MessagingService.Publisher.Abstraction;
+using DigitalData.MessagingService.RabbitMQ;
+using Microsoft.AspNetCore.DataProtection;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace DigitalData.MessagingService.Infrastructure;
+
+/// <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 AddInfrastructure(
+        this IServiceCollection services,
+        IConfiguration configuration)
+    {
+        // --- External Services ---
+        // Email Service (using Limilabs Mail.dll - Singleton for use in EmailSenderWorker)
+        services.AddSingleton<IEmailService, LimilabsEmailService>();
+        
+        // PDF Processing Service (using DevExpress.Pdf)
+        services.AddScoped<IPdfProcessingService, DevExpressPdfProcessingService>();
+        
+        // Encryption Service (using Data Protection API - Singleton, thread-safe)
+        services.AddSingleton<IEncryptionService, DataProtectionEncryptionService>();
+
+        // --- Email Queue (RabbitMQ) ---
+        services.AddSingleton<OutgoingEmailConsumer>();
+        services.AddSingleton<IOutgoingEmailPublisher, OutgoingEmailPublisher>();
+
+        // --- RabbitMQ Configuration ---
+        services.AddRabbitMqConnectionFactory(configuration);
+
+        // --- Data Protection (for encryption) ---
+        services.AddDataProtection()
+            .PersistKeysToFileSystem(new DirectoryInfo(@"C:\ProgramData\EmailService\Keys"))
+            .SetApplicationName("MessagingService");
+
+        // Register Background Workers
+        services.AddHostedService<AsyncInitWorker>();
+
+        return services;
+    }
+}