Support Map field types

Problem/Motivation

The core's "map" DataType and related field seem to be unsupported by default content and thus ignored when exporting entities.

Steps to reproduce

Have a "map" field on an entity, i.e.:

    $fields['settings'] = BaseFieldDefinition::create('map')
      ->setLabel(t('Settings'))
      ->setDescription(t('Arbitrary settings defined by plugins.'))
      ->setDisplayOptions('form', [
        'type' => 'assessment_answer_settings',
        'weight' => -5,
      ]);

Have some entities with values for this field.

Try to export them using default content (drush default-content:export)

The generated yml is missing the value for this field.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes