Added configuration for additional known files in project root

The additional_known_files_in_project_root configuration option allows site maintainers to list non-core files that need to be included in an automatic update. This allows specific files such as patches.json to be included. Previously the only way to include non-core files in an update was to disable filtering unknown files.

Before:
\Drupal::config('package_manager.settings)->set('include_unknown_files_in_project_root', TRUE)->save()

After:
\Drupal::config('package_manager.settings)->set('additional_known_files_in_project_root', ['patches.json', 'patches.lock.json'])->save()