feat(client): add MessagingServiceClient and dependency injection extensions

Introduce the Email contract and MessagingServiceClient that send mail through the Application layer via MediatR, and add AddMessagingServiceClient overloads for delegate- and IConfiguration-based setup. Remove the empty client DI placeholders.

src/presentation/DigitalData.MessagingService.Client/MessagingServiceClient.cs

@@ -0,0 +1,34 @@
+#if NET
+using DigitalData.MessagingService.Application.EmailAccounts.Queries;
+using DigitalData.MessagingService.Application.EmailSending.Commands;
+using MediatR;
+
+namespace DigitalData.MessagingService.Client;
+
+/// <summary>
+/// 
+/// </summary>
+public class MessagingServiceClient(ISender sender)
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="email"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public async Task SendAsync(Email email, CancellationToken cancellationToken = default)
+    {
+        await sender.Send(new PublishEmailViaSmtpCommand()
+        {
+            Sender = new GetEmailAccountQuery()
+            {
+                Username = email.From
+            },
+            Recipients = email.To,
+            Subject = email.Subject,
+            Body = email.Body,
+            IsHtml = email.IsHtml
+        }, cancellationToken);
+    }
+}
+#endif