Don't allow arbitrary kwargs in options

[jyu00]

What is the expected feature or enhancement?

Each of the classes in options is decorated with @_flexible, which "is used to dynamically create a new dataclass with the arbitrary kwargs input". This was done because the options were still in flux at the time.

However, validation was added to disallow arbitrary kwargs input (for example), which contradicts with _flexible.

Since options are more settled, we don't really need to force option classes to accept arbitrary kwargs. If we remove that, we also don't need this additional validation, since dataclass would take care of that for us.

We can still allow arbitrary kwargs in run() that will be merged into the final dictionary.

We still need the part of _flexible that converts input dictionary to objects.

Acceptance criteria

Options classes no longer need to accept arbitrary kwargs, which can still be specified in run().