feat(application): add IMessagingServiceClient abstraction

The client package will expose two interchangeable implementations (in-process and HTTP), so the contract moves into the application layer where both can depend on it. SendAsync returns the queued message id.

src/core/DigitalData.MessagingService.Application/Common/Interfaces/IMessagingServiceClient.cs

@@ -0,0 +1,19 @@
+#if NET
+using DigitalData.MessagingService.Application.EmailSending.Commands;
+
+namespace DigitalData.MessagingService.Application.Common.Interfaces;
+
+/// <summary>
+/// 
+/// </summary>
+public interface IMessagingServiceClient
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="command"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public Task<Guid> SendAsync(PublishEmailViaSmtpCommand command, CancellationToken cancellationToken = default);
+}
+#endif