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/EmailSending/Commands/SendEmailCommand.cs

@@ -0,0 +1,47 @@
+using AutoMapper;
+using DigitalData.MessagingService.Publisher.Abstraction;
+using MediatR;
+
+namespace DigitalData.MessagingService.Application.EmailSending.Commands;
+
+/// <summary>
+/// Command to send an email (enqueue to RabbitMQ)
+/// </summary>
+public record SendEmailCommand : IRequest<OutgoingEmailEvent>
+{
+    /// <summary>
+    /// Recipient email address
+    /// </summary>
+    public required string Recipient { get; init; }
+
+    /// <summary>
+    /// Email subject
+    /// </summary>
+    public required string Subject { get; init; }
+
+    /// <summary>
+    /// Email body (HTML or plain text)
+    /// </summary>
+    public required string Body { get; init; }
+
+    /// <summary>
+    /// Is HTML email (default: true)
+    /// </summary>
+    public bool IsHtml { get; init; } = true;
+}
+
+/// <summary>
+/// Handler for SendEmailCommand
+/// Creates EmailOutbox entity via AutoMapper and enqueues to RabbitMQ
+/// </summary>
+public class SendEmailCommandHandler(IOutgoingEmailPublisher Publisher, IMapper Mapper) : IRequestHandler<SendEmailCommand, OutgoingEmailEvent>
+{
+    public async Task<OutgoingEmailEvent> Handle(SendEmailCommand request, CancellationToken cancellationToken)
+    {
+        var outgoingEmailEvent = Mapper.Map<OutgoingEmailEvent>(request);
+
+        // Enqueue to RabbitMQ
+        await Publisher.EnqueueAsync(outgoingEmailEvent, cancellationToken);
+        return outgoingEmailEvent;
+    }
+}