Consider making IValueDescriptor.GetDefaultValue() easier to use with options

Hey there! I'm writing some command examples for custom help output, and I had a little difficulty finding the default value for an option from the ParseResult. This is what I ended up with:

```c#
if(((IValueDescriptor)opt).HasDefaultValue)
object? value = ((IValueDescriptor)opt).GetDefaultValue();
```

It would be very helpful I was not required to cast the option, and the method didn't throw an exception if there was no default. In that case, I could get the default value by writing something like:

```c#
object? value = opt.GetDefaultValue();
```

Arguments already make the method and field available. It would still be helpful to have this functionality for options too, even if it isn't possible to get around the exception.