feat: add callback to sender

also done:

• MininmalStraightForwardTest to send a mininmal submission
• moved interfaces to own files
• removed CallbackDto
• changed Callback from record to class

contributes to planning#1096 (closed) for the .NET SDK

Examples/ConsoleAppExample/SenderDemo.cs

@@ -48,6 +48,7 @@ public static class SenderDemo {
             .SetPaymentInformation(new PaymentInformation(PaymentMethod.Creditcard,
                 PaymentStatus.Booked, "12345445", "2345566"))
             .SetReplyChannel(ReplyChannel.FromEMail("a@example.com"))
+            .SetCallback(new Uri("https://google.com"), "TopSecretAtLeast32CharactersLong")
             .Build();
 
         var sender =
@@ -92,6 +93,7 @@ public static class SenderDemo {
             .SetEncryptedMetadata(encryptedMetadata!)
             .SetEncryptedData(encryptedData!)
             .AddEncryptedAttachments(encryptedAttachments)
+            .SetCallback(new Uri("https://google.com"), "TopSecretAtLeast32CharactersLong")
             .Build();
         var sentSubmission = await ClientFactory
             .GetSenderClient(FitConnectEnvironment.Test, clientId, clientSecret, logger)