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/DigitalData.MessagingService.Publisher.Abstraction.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+	<TargetFrameworks>net462;net480;net8.0</TargetFrameworks>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+  </PropertyGroup>
+
+</Project>

DigitalData.MessagingService.Publisher/IOutgoingEmailPublisher.cs

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+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);
+}

DigitalData.MessagingService.Publisher/OutgoingEmailEvent.cs

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+using System;
+
+namespace DigitalData.MessagingService.Publisher.Abstraction;
+
+public class OutgoingEmailEvent
+{
+    public Guid Id { get; set; }
+
+    /// <summary>
+    /// Recipient email address
+    /// </summary>
+    public string Recipient { get; set; } = null!;
+
+    /// <summary>
+    /// Email subject
+    /// </summary>
+    public string Subject { get; set; } = null!;
+
+    /// <summary>
+    /// Email body (HTML or plain text)
+    /// </summary>
+    public string Body { get; set; } = null!;
+
+    /// <summary>
+    /// Is HTML email (default: true)
+    /// </summary>
+    public bool IsHtml { get; set; } = true;
+
+    public DateTime QueuedAt { get; set; }
+}