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/FitConnect.csproj

@@ -86,6 +86,7 @@
         </Compile>
     </ItemGroup>
 
+
     <Target Name="PreBuild" BeforeTargets="PreBuildEvent">
     </Target>