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.Infrastructure/Persistence/MessagingServiceDbContext.cs

@@ -0,0 +1,11 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace DigitalData.MessagingService.Infrastructure.Persistence;
+
+/// <summary>
+/// Entity Framework Core DbContext for MessagingService.
+/// IMPORTANT: This context maps to a LEGACY database - NO schema modifications allowed!
+/// </summary>
+public class MessagingServiceDbContext(DbContextOptions<MessagingServiceDbContext> options) : DbContext(options)
+{
+}