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/presentation/DigitalData.MessagingService.API/Controllers/EmailsController.cs

@@ -0,0 +1,36 @@
+using DigitalData.MessagingService.Application.EmailSending.Commands;
+using MediatR;
+using Microsoft.AspNetCore.Mvc;
+
+namespace DigitalData.MessagingService.API.Controllers;
+
+/// <summary>
+/// Email sending API controller
+/// Enqueues outgoing emails to RabbitMQ for async processing
+/// </summary>
+[ApiController]
+[Route("api/[controller]")]
+public class EmailsController(IMediator mediator) : ControllerBase
+{
+    /// <summary>
+    /// Send email (enqueue to RabbitMQ for background processing)
+    /// </summary>
+    /// <param name="command">Send email command</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>HTTP 202 Accepted (queued for processing)</returns>
+    [HttpPost("send")]
+    [ProducesResponseType(StatusCodes.Status202Accepted)]
+    [ProducesResponseType(StatusCodes.Status400BadRequest)]
+    public async Task<IActionResult> SendEmail([FromBody] SendEmailCommand command, CancellationToken cancellationToken)
+    {
+        var outgoingEmailEvent = await mediator.Send(command, cancellationToken);
+
+        return Accepted(new
+        {
+            CommandId = outgoingEmailEvent.Id,
+            To = outgoingEmailEvent.Recipient,
+            outgoingEmailEvent.Subject,
+            outgoingEmailEvent.QueuedAt
+        });
+    }
+}