Add default config & config schema

The config schema for required_plugin_options will have to be provided by the respective modules. This is what it looks like in my WIP required_by_content_moderation_state module:

# config/schema/required_by_content_moderation_state.schema.yml
required_api.plugin_options.required_by_content_moderation_state:
  type: sequence
  label: 'Content moderation states'
  sequence:
    type: string
    label: 'Content moderation state'