Enhance serialization support for class interfaces in SelfSerializingNormalizer

[gehrisandro]

Edit: Support for union types added

This PR enhances the SelfSerializingNormalizer to support deserializing of properties which are a union type or a type of interface extending SerializedByVerbs.

Before, it was not possible to (de)serialize the following events:

class EventWithInterfaceType extends Event
{
    public function __construct(
        public DataInterface $data,
    ) {}
}

// or

class EventWithUnionType extends Event
{
    public function __construct(
        public StringData|NumberData $data,
    ) {}
}

As context, the interface and DTO declarations:

interface DataInterface extends SerializedByVerbs {}

class StringData implements DataInterface
{
    use NormalizeToPropertiesAndClassName;

    public string $value;
}

class NumberData implements DataInterface
{
    use NormalizeToPropertiesAndClassName;

    public int $value;
}

This change is non-breaking.

If you prefer a different approach to implement the behaviour, please let me know.

[gehrisandro]

Hi @inxilpro @DanielCoulbourne

Any chance to have a look on this PR? It is only a small change to the SelfSerializingNormalizer which allows deserializing properties with union or interface types.