feat(transformer): allow to pass invokable object in transformer

[joelwurtz]

It allow having :

class Foo
{
    #[MapTo(transformer: new CustomTransformer())]
    public string $foo;
}

class CustomTransformer
{
    public function __invoke($value, $source, $context)
    {
       // ...
       return $transformedValue;
    }
}

[joelwurtz]

I'm not sure how we should handle this with the already existing PropertyTransformerInterface which is done when we want to pass a service (with other dependencies).

Should we merge both cases into a single interface ?

[Korbeil]

Don't think we need to merge the interface with it now