Please provide a Source Generator that generates an implementation of ICommandHandler.

I would like a Source Generator that supports implementing ICommandHandler like DragonFruit.

For example, for the following code,

```cs
partial class CommandHandler
{
[CommandHandler]
Task Handle(
int foo)
{
// command handling
}
}
```

generates following implementation.

```cs
partial class CommandHandler : ICommandHandler
{
public Task InvokeAsync(InvocationContext context)
{
var fooOption = context.ParseResult.CommandResult.Command.Options
.SingleOrDefault(static option => option.Name == "foo");

var fooValue = context.ParseResult.GetValueForOption(fooOption);

return this.Handle(fooValue);
}
}
```

The above code is for a very simple case; more considerations are needed for what actually works.