Add dispose in snippets (dotnet#10242)

Author: BlueBles

Diff for: snippets/cpp/VS_Snippets_Remoting/NclMailASync/cpp/mailasync.cpp

@@ -82,6 +82,7 @@ int main(array<String^>^ args)
         }
         // Clean up.
         delete message;
+        delete client;
         client = nullptr;
         Console::WriteLine("Goodbye.");
     }

Diff for: snippets/csharp/System.Net.Mail/MailAddress/.ctor/mailasync.cs

@@ -74,6 +74,7 @@ public static void Main(string[] args)
             }
             // Clean up.
             message.Dispose();
+            client.Dispose();
             Console.WriteLine("Goodbye.");
         }
     }

Diff for: snippets/visualbasic/VS_Snippets_Remoting/NclMailASync/vb/mailasync.vb

@@ -63,6 +63,7 @@ Namespace Examples.SmtpExamples.Async
             End If
             ' Clean up.
             message.Dispose()
+            client.Dispose()
             Console.WriteLine("Goodbye.")
         End Sub
     End Class