Add RabbitMQ-based email publisher and DI support

Introduced `OutgoingEmailPublisher` for RabbitMQ-based email
queueing with message persistence, scalability, and reliability.
Added dependency injection support via `AddMessagingServicePublisher`
extension method. Enhanced RabbitMQ topology setup with exchanges,
queues, and Dead Letter Queues (DLQ).

Updated `DigitalData.MessagingService.Publisher.csproj` and
`DigitalData.MessagingService.RabbitMQ.csproj` to support
`net462`, `net480`, and `net8.0`. Added project references
and conditional package references for compatibility.

Integrated logging with `Microsoft.Extensions.Logging` and
used `System.Text.Json` for serialization. Implemented lazy
initialization for RabbitMQ channels to improve performance.

src/infrastructure/DigitalData.MessagingService.Publisher/DependencyInjection.cs

@@ -0,0 +1,38 @@
+using DigitalData.MessagingService.Publisher.Abstraction;
+using DigitalData.MessagingService.RabbitMQ;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace DigitalData.MessagingService.Publisher;
+
+public static class DependencyInjection
+{
+    public static IServiceCollection AddMessagingServicePublisher(this IServiceCollection services, Action<Configuration>? configure = null)
+    {
+        if(configure is not null)
+        {
+            var configuration = new Configuration(services);
+            configure(configuration);
+        }
+
+        // --- Email Queue (RabbitMQ) ---
+        services.AddSingleton<IOutgoingEmailPublisher, OutgoingEmailPublisher>();
+
+        return services;
+    }
+
+    public class Configuration
+    {
+        private readonly IServiceCollection _services;
+        internal Configuration(IServiceCollection services)
+        {
+            _services = services;
+        }
+
+        public Configuration AddRabbitMqConnectionFactory(IConfiguration configuration)
+        {
+            _services.AddRabbitMqConnectionFactory(configuration);
+            return this;
+        }
+    }
+}