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/Common/Interfaces/IEncryptionService.cs

@@ -0,0 +1,10 @@
+namespace DigitalData.MessagingService.Application.Common.Interfaces;
+
+/// <summary>
+/// Encryption service interface for password encryption.
+/// </summary>
+public interface IEncryptionService
+{
+    string Encrypt(string plainText);
+    string Decrypt(string cipherText);
+}