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.Application/DependencyInjection.cs

@@ -0,0 +1,40 @@
+using System.Reflection;
+using FluentValidation;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace DigitalData.MessagingService.Application;
+
+/// <summary>
+/// Dependency injection configuration for Application layer.
+/// </summary>
+public static class DependencyInjection
+{
+    public static IServiceCollection AddApplicationServices(this IServiceCollection services, IConfiguration configuration)
+    {
+        var assembly = Assembly.GetExecutingAssembly();
+
+        // Read LuckyPennySoft license key from appsettings.json
+        var licenseKey = configuration.GetValue<string>("LuckyPennySoftLicenseKey")
+            ?? throw new InvalidOperationException("LuckyPennySoftLicenseKey not found in configuration");
+
+        // MediatR - Register all handlers
+        services.AddMediatR(config =>
+        {
+            config.LicenseKey = licenseKey;
+            config.RegisterServicesFromAssembly(assembly);
+        });
+
+        // AutoMapper - Use built-in DI extension (AutoMapper 16.2.0+)
+        services.AddAutoMapper(config =>
+        {
+            config.LicenseKey = licenseKey;
+            config.AddMaps(assembly);
+        });
+
+        // FluentValidation - Register all validators
+        services.AddValidatorsFromAssembly(assembly);
+
+        return services;
+    }
+}