This method is called by the REST input parsers to allow a field type to
pre process the given $incomingSettingsHash before it is handled by
{@link eZ\Publish\SPI\FieldType\FieldType::fieldSettingsFromHash()}. The
$incomingSettingsHash can be expected to conform to the rules that
need to apply to hashes accepted by fieldSettingsFromHash(). The return
value of this method replaces the $incomingSettingsHash.