Eliminate need of ServiceProduct

[chuwy]

@Baccata has mentioned we can use Scala 3 macros to get rid need of generateServiceProduct trait (and hence an explicit TasksServiceGen.serviceProduct argument).

My only concern here is that I'm bit afraid of using macros against codegenerated code as a macro will expect code of very specific structure and it should be easy to break it by tiny upstream Smithy4s changes.