Issue #2206687 by damiankloip: Replace user_authenticate with a UserAuth service.

core/modules/basic_auth/basic_auth.services.yml

...b/Drupal/basic_auth/Authentication/Provider/BasicAuth.php

core/modules/user/lib/Drupal/user/Form/UserLoginForm.php

core/modules/user/lib/Drupal/user/UserAuth.php

core/modules/user/lib/Drupal/user/UserAuthInterface.php

core/modules/user/tests/Drupal/user/Tests/UserAuthTest.php

core/modules/user/user.module

core/modules/user/user.services.yml