[D8] Form API: Introduce proper #types for 'option' and 'optgroup', and make #options consistent.

This was basically triggered by this forum post: https://forum.backdropcms.org/forum/how-set-selected-tag-form-select-opt...

As it is currently, you cannot easily add attributes to individual options of select elements, and it requires either a lot of theming work, or to port a module like https://www.drupal.org/project/form_options_attributes. Even with that module, things are not consistent, so we should consider backporting the D8 solution being worked in https://www.drupal.org/node/342316