Previously field settings were normalized at preSave stage. Now, normalization happens also when fields are being created. Normalization means making a union of the provided settings with default settings, and removing any unsupported options (options not present in default settings). This change affects field_storage_config, field_config, and base_field_override entities, which are used to define fields.
On top of the regular normalization, entity reference fields have an extra step. In this extra step, the 'handler' setting is normalized into a fully qualified plugin id for the derivative of the selected target type.
These changes are introduced to help with future work on config validation #3324140: Convert field_storage_config and field_config's form validation logic to validation constraints and with improvements to the Field UI user experience: #3358049: Save FieldStorageConfig at the same time as FieldConfig.
