Use third_party_setting to store custom field's configuration.

Problem/Motivation

3291801 extends config storage but I believe any custom config should be stored in third_party_settings.

Data model changes

* store custom data under third_party_settings, create deploy script
* update all places where we use field's settings
* remove svg_image_config_schema_info_alter function
* create config/schema/svg_image.schema.yml , something similar to https://www.drupal.org/docs/8/modules/webform/webform-cookbook/providing... →