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/RabbitMqConfiguration.cs

@@ -0,0 +1,55 @@
+namespace DigitalData.MessagingService.RabbitMQ
+{
+    /// <summary>
+    /// Configuration for RabbitMQ connection
+    /// </summary>
+    public class RabbitMqConfiguration
+    {
+        /// <summary>
+        /// Configuration section name in appsettings.json
+        /// </summary>
+        public const string SectionName = "RabbitMQ";
+
+        /// <summary>
+        /// RabbitMQ server hostname
+        /// </summary>
+        public string HostName { get; set; } = "localhost";
+
+        /// <summary>
+        /// RabbitMQ AMQP port (default: 5672)
+        /// </summary>
+        public int Port { get; set; } = 5672;
+
+        /// <summary>
+        /// RabbitMQ username
+        /// </summary>
+        public string UserName { get; set; } = "guest";
+
+        /// <summary>
+        /// RabbitMQ password
+        /// </summary>
+        public string Password { get; set; } = "guest";
+
+        /// <summary>
+        /// Virtual host (default: /)
+        /// </summary>
+        public string VirtualHost { get; set; } = "/";
+
+        /// <summary>
+        /// Enable automatic recovery on connection failure
+        /// </summary>
+        public bool AutomaticRecoveryEnabled { get; set; } = true;
+
+        /// <summary>
+        /// Network recovery interval in seconds
+        /// </summary>
+        public int NetworkRecoveryIntervalSeconds { get; set; } = 10;
+
+        public string QueueName { get; set; }
+        public string ExchangeName { get; set; }
+        public string RoutingKey { get; set; }
+        public string DlqQueueName { get; set; }
+        public string DlqExchangeName { get; set; }
+        public string DlqRoutingKey { get; set; }
+    }
+}