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/Mappings/EmailMappingProfile.cs

@@ -0,0 +1,19 @@
+using AutoMapper;
+using DigitalData.MessagingService.Application.EmailSending.Commands;
+using DigitalData.MessagingService.Publisher.Abstraction;
+
+namespace DigitalData.MessagingService.Application.Common.Mappings;
+
+/// <summary>
+/// AutoMapper profile for Emails
+/// </summary>
+public class EmailMappingProfile : Profile
+{
+    public EmailMappingProfile()
+    {
+        // SendEmailCommand -> OutgoingEmailEvent
+        CreateMap<SendEmailCommand, OutgoingEmailEvent>()
+            .ForMember(dest => dest.Id, opt => opt.MapFrom(_ => Guid.NewGuid()))
+            .ForMember(dest => dest.QueuedAt, opt => opt.MapFrom(_ => DateTime.Now));
+    }
+}