Refactor: Consolidate Publisher.Abstraction into Abstraction

The `DigitalData.MessagingService.Publisher.Abstraction` project has been removed, and its functionality has been merged into a new project named `DigitalData.MessagingService.Abstraction`.

- Updated namespaces from `Publisher.Abstraction` to `Abstraction` across all relevant files, including DTOs, interfaces, and classes.
- Modified the solution file to remove `Publisher.Abstraction` and add `Abstraction`, updating solution configurations and nested project mappings.
- Replaced project references to `Publisher.Abstraction` with `Abstraction` in all affected project files.
- Updated tests and integration tests to reflect the namespace and project changes.
- Refactored application-level files such as `EmailMappingProfile` and `DependencyInjection.cs` to use the new namespace.

This refactor simplifies the project structure and ensures consistency across the solution.

tests/DigitalData.MessagingService.Tests/Integration/OutgoingEmailPublisherTests.cs

@@ -1,7 +1,7 @@
 using System.Text;
 using System.Text.Json;
 using DigitalData.MessagingService.Publisher;
-using DigitalData.MessagingService.Publisher.Abstraction;
+using DigitalData.MessagingService.Abstraction;
 using DigitalData.MessagingService.RabbitMQ;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;