fix: signature of Map attributes `transformer` property

[JorickPepin]

According to the PropertyTransformerInterface, the signature of a transformer should be:

/**
 * @param mixed                       $value   the value of the property to transform, can be null if there is no way to read the data from the mapping
 * @param object|array<string, mixed> $source  the source input on which the custom transformation applies
 * @param array<string, mixed>        $context Context during mapping
 */
public function transform(mixed $value, object|array $source, array $context): mixed;

[joelwurtz]

Thanks @JorickPepin