Implement Plunk email sending service with HttpClient integration and DTO definition.

API/Models/Outgoing/PlunkEmailDto.cs

@@ -0,0 +1,9 @@
+namespace API.Models.Outgoing;
+
+public class PlunkEmailDto
+{
+    public required string To { get; set; }
+    public required string Subject { get; set; }
+    public required string Body { get; set; }
+    public string? Name { get; set; }
+}