Refactor and add OutgoingEmailCreateDto record

Introduced a new namespace `DigitalData.MessagingService.Publisher.Abstraction` in `OutgoingEmailCreateDto.cs` and `OutgoingEmailEvent.cs`.

Added a new record `OutgoingEmailCreateDto` with properties for recipient, subject, body, HTML flag, and queue timestamp.

Converted `OutgoingEmailEvent` from a class to a record.

Removed the unnecessary `using System;` directive from `OutgoingEmailEvent.cs`.

src/core/DigitalData.MessagingService.Publisher.Abstraction/OutgoingEmailCreateDto.cs

@@ -0,0 +1,26 @@
+namespace DigitalData.MessagingService.Publisher.Abstraction;
+
+public record OutgoingEmailCreateDto
+{
+    /// <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; }
+}

src/core/DigitalData.MessagingService.Publisher.Abstraction/OutgoingEmailEvent.cs

@@ -1,8 +1,6 @@
-using System;
+namespace DigitalData.MessagingService.Publisher.Abstraction;
 
-namespace DigitalData.MessagingService.Publisher.Abstraction;
-
-public class OutgoingEmailEvent
+public record OutgoingEmailEvent
 {
     public Guid Id { get; set; }