Added the ability to specify the resource type in the resource data

[KorDum]

This change includes two improvements:

1. According to the JSON:API specification, the resource type is always a string (implying a non-empty string), which means that null is not allowed in the resource type.
2. For implementing complex scenarios with Discriminator Map, it is more logical to embed the type at the resource data level, especially since the method contract allows you to not explicitly specify the resource type when forming the resource. See below for details.

Now you can implement scenarios like these:

final class FooTransformer extends TransformerAbstract
{
    public function transform(object $obj): array
    {
        return [
            'id' => '1',
            'type' => 'foo',
        ];
    }
}

final class BarTransformer extends TransformerAbstract
{
    public function transform(object $obj): array
    {
        return [
            'id' => '2',
            'type' => 'bar',
        ];
    }
}

final class MixedTransformer extends TransformerAbstract
{
    public function transform(object $obj): array
    {
        return match (true) {
            $obj instanceof Foo => new FooTransformer()->transform($obj),
            $obj instanceof Bar => new BarTransformer()->transform($obj),
            default => throw new InvalidArgumentException(),
        };
    }
}

{"data":[
    {"type":"foo","id":"1","attributes":{}},
    {"type":"bar","id":"2","attributes":{}}
]}

Also works with includes.

I think this change breaks backward compatibility and should be highlighted somehow.