What about providing serialization layer instead of extensions?

Create simple interface like this https://github.com/php-enqueue/enqueue-dev/blob/2b1094c3ff6db885cc999ce15291f598f352da6a/pkg/rdkafka/Serializer.php and ship lib with 2 serializers: json (simple, native like now) and Symfony's serializer.

It would introduce one extra concept, and it would be possible to achieve the goal with extensions. There might be an extension that supports Symfony serializer.

closed in favor #517