Add Commands Explicitly to ServiceCollection

Hello @csharpfritz .
Right now all `IBasicCommand` and `IExtendedCommand` are got via scanning Assembly and added to `IServiceCollection` with singleton lifetime.
https://github.com/csharpfritz/Fritz.StreamTools/blob/78587eb030cf08ffc479079b858b8b89d2bf7d97/Fritz.Chatbot/FritzBot.cs#L63-L80
I'm suggesting changing this behavior to adding commands explicitly in startup for two reasons :

1. In order to inject a scoped service like `DbContext`, the command needs to be scoped or transient.
2. We could prevent adding some command in startup based on configuration and prevent running a command (even `CanExecute`).