Allow overriding cached values for specific operations to make it possible to use the same form class for multiple steps

Currently, when using the Wizard API, you can specify the same form class in multiple operations (in ::getOperations()), but all those steps will work identically.

To facilitate code reuse, it'd be great pass a set of overrides for the wizard's cached values when on a particular step. This way, you can use the same form class, but it'll operate on different data. You can even have the values be dependent on other dynamic data in the wizard!

Here's how it'd be used:

class MyWizard extends FormWizardBase {
  public function getOperations($cached_values) {
    return [
      'one' => [
        'title' => $this->t('Step One'),
        'form' => '\Drupal\my_module\StepForm',
        // On this step, $cached_values['value'] will equal 'one'.
        'values' => ['value' => 'one'],
      ],
      'two' => [
        'title' => $this->t('Step Two'),
        'form' => '\Drupal\my_module\StepForm',
        // On this step, $cached_values['value'] will equal 'two'.
        'values' => ['value' => 'two'],
      ],
    ];
  }
}