Constructs the block configuration form.
This method allows base implementations to add a generic configuration form for extending block plugins.
array $form: The form definition array for the block configuration form.
array $form_state: An array containing the current state of the configuration form.
array $form The renderable form array representing the entire configuration form.
\Drupal\block\BlockFormController::form()
\Drupal\block\BlockInterace::validate()
\Drupal\block\BlockInterace::submit()
public function form($form, &$form_state);