Add Publisher.Abstraction project and refactor namespaces

Introduced a new project, `DigitalData.MessagingService.Publisher.Abstraction`, to encapsulate the `OutgoingEmailEvent` class and `IOutgoingEmailPublisher` interface. The project targets multiple frameworks (`net462`, `net480`, `net8.0`) and enables nullable reference types and the latest C# language version.

Moved `OutgoingEmailEvent` and `IOutgoingEmailPublisher` to the new project, updating their namespaces and replacing `required` properties with mutable ones in `OutgoingEmailEvent`. Updated all dependent files to reference the new namespace.

Updated the solution file to include the new project and adjusted build configurations. Cleaned up unused `using` directives and removed redundant `LangVersion` property in the new project file.

DigitalData.MessagingService.Publisher/IOutgoingEmailPublisher.cs

@@ -0,0 +1,14 @@
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace DigitalData.MessagingService.Publisher.Abstraction;
+
+/// <summary>
+/// Email queue interface for outgoing emails.
+/// </summary>
+public interface IOutgoingEmailPublisher
+{
+    Task EnqueueAsync(OutgoingEmailEvent outgoingEmailEvent, CancellationToken cancellationToken = default);
+
+    Task<int> GetQueueDepthAsync(CancellationToken cancellationToken = default);
+}