DX improvements to the Simple Widget State and it's accessibility

Problem/Motivation

Currently the IEF simple widget does not provide an 'instance' property in the form_state storage (like the complex widget does) and doesn't make it's `ief_id` value availible to the `inline_entity_form` element it builds (like the complex widget does).

This means that it is imposible to reference the correct widget state for an simple `inline_entity_form` in `hook_inline_entity_form_entity_form_alter`, and even if you can there if no information about the inplimenting field avalible as the 'instance' propery is missing.

Proposed resolution

Allow the IEF Simple widget to set the correct ief_id on the `inline_entity_form` element and provide a 'instance' property in the widget storage.

Once the cahnges have been implements developers will be able to do the following:

/**
 * Implements hook_inline_entity_form_entity_form_alter()
 */
function hook_inline_entity_form_entity_form_alter(array &$entity_form, FormStateInterface &$form_state) {
  // Currently, can't do this for IEF Simple widgets
  $widget_state = $form_state->get(['inline_entity_form', $entity_form['#ief_id']]);

  // Currently, no easy way to access the definition of the field responsible for this inline_entity_form
  $field_definition = $form_state->get(['inline_entity_form', $entity_form['#ief_id'], 'instance']);
  if ($field_definition->getName() === 'field_custom') {
    // Do stuff..
  }
}