Add Commands Explicitly to ServiceCollection

[Kahbazi]

Hello @csharpfritz .
Right now all IBasicCommand and IExtendedCommand are got via scanning Assembly and added to IServiceCollection with singleton lifetime.

Fritz.StreamTools/Fritz.Chatbot/FritzBot.cs

Lines 63 to 80 in 78587eb

	public static void RegisterCommands(IServiceCollection services)
	{
	// Register basic commands
	foreach (var type in typeof(FritzBot).Assembly.GetTypes()
	.Where(t => typeof(IBasicCommand)
	.IsAssignableFrom(t) && !t.IsAbstract && t.IsClass))
	{
	services.AddSingleton(typeof(IBasicCommand), type);
	}
	// Register extended commands
	foreach (var type in typeof(FritzBot).Assembly.GetTypes()
	.Where(t => typeof(IExtendedCommand)
	.IsAssignableFrom(t) && !t.IsAbstract && t.IsClass))
	{
	services.AddSingleton(typeof(IExtendedCommand), type);
	}
	}

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).