public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {

  $properties = parent::propertyDefinitions($field_definition);
  $properties['icon_id'] = DataReferenceDefinition::create('entity')
    ->setLabel('Image')
    ->setDescription(new TranslatableMarkup('The referenced entity image'))
    ->setComputed(TRUE)
    ->setReadOnly(FALSE)
    ->setTargetDefinition(EntityDataDefinition::create('media'));

  return $properties;
}

public static function schema(FieldStorageDefinitionInterface $field_definition) {
    $schema = parent::schema($field_definition);

    $schema['columns']['icon_id'] = [
      'description' => 'The icon ID',
      'type' => 'int',
      'unsigned' => TRUE,
    ];

    return $schema;
  }