Issue #3490549 by kim.pepper, lendude, larowlan: Add an interface for FileUploadHandler

...ib/Drupal/Core/File/Event/FileUploadSanitizeNameEvent.php

...les/ckeditor5/src/Controller/CKEditor5ImageController.php

core/modules/file/file.module

core/modules/file/file.services.yml

core/modules/file/src/Upload/FileUploadHandler.php

core/modules/file/src/Upload/FileUploadHandlerInterface.php

core/modules/jsonapi/src/Controller/FileUpload.php