In this method the field type puts the data which is stored in the field of content in the repository
into the property FieldValue::data. The format of $data is a primitive, an array (map) or an object, which
is then canonically converted to e.g. json/xml structures by future storage engines without
further conversions. For mapping the $data to the legacy database an appropriate Converter
(implementing eZ\Publish\Core\Persistence\Legacy\FieldValue\Converter) has implemented for the field
type. Note: $data should only hold data which is actually stored in the field. It must not
hold data which is stored externally.
The $externalData property in the FieldValue is used for storing data externally by the
FieldStorage interface method storeFieldData.
The FieldValuer::sortKey is build by the field type for using by sort operations.