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

FitConnect/EncryptedSender.cs

@@ -18,6 +18,7 @@ public class EncryptedSubmissionBuilder : IEncryptedBuilderWithData,
 
     public SendableEncryptedSubmission Build() {
         _submission.SchemaUri = SchemaUri;
+        _submission.Callback = Callback;
         return _submission;
     }