CheckboxValidation

A DataValidation for a `https://developers.google.com/apps-script/reference/forms/checkbox-item`.

```javascript
// Add a checkBox item to a form and require exactly two selections.
const form = FormApp.create('My Form');
const checkBoxItem = form.addCheckboxItem();
checkBoxItem.setTitle('What two condiments would you like on your hot dog?');
checkBoxItem.setChoices([
  checkBoxItem.createChoice('Ketchup'),
  checkBoxItem.createChoice('Mustard'),
  checkBoxItem.createChoice('Relish'),
]);
const checkBoxValidation = FormApp.createCheckboxValidation()
                               .setHelpText('Select two condiments.')
                               .requireSelectExactly(2)
                               .build();
checkBoxItem.setValidation(checkBoxValidation);
```