Provides a search form for site wide search.
Search plugins can define method searchFormAlter() to alter the form. If they have additional or substitute fields, they will need to override the form submit, making sure to redirect with a GET parameter of 'keys' included, to trigger the search being processed by the controller, and adding in any additional query parameters they need to execute search.
Hierarchy
class \Drupal\Core\Form\FormBase implements ContainerInjectionInterface, FormInterface