Issue #3620862 by luissousa21: Add the missing configuration schema.

Closes #3620862

Adds config/schema/media_pdf_thumbnail.schema.yml for media_pdf_thumbnail.settings and for the image formatter, which extends field.formatter.settings.image. The per-bundle keys are named after the media bundle, so they cannot go in the YAML; hook_config_schema_info_alter() adds them, and hook_ENTITY_TYPE_insert()/_delete() on media_type clear the typed config definitions so a bundle added later is covered in the same request. Six settings hold a different PHP type depending on whether the value came from getDefaultSettings() or from a form save, so they are typed ignore; details and the follow-up in the issue. Five functional tests, all failing on unpatched 7.0.0.