Add checkboxes/radios input widgets

- Follow the example of the text input widget, see \Drupal\typed_data\Plugin\TypedDataFormWidget\TextInputWidget and \Drupal\Tests\typed_data\Functional\TypedDataFormWidget\TextInputWidgetTest
- Add two plugins, "radios" and "checkboxes". Both can extend from the same base class if helpful.
- Add test cases.

I think having two plugins with proper names is better than one plugin that can handle both but has a confusing name. So let's better do both plugins and implement isApplicable accordingly.