Add support for using nested DTOs

[norkunas]

Continuation of #6709 and #6574

I've cherry picked commits from these and rebased.
One test is commented with scalar results because I don't think that it makes sense to returns object when you expect scalars?

[derrabus]

Thank you. Can you please have a look at the errors reported by our static analysis?

[norkunas]

Thank you. Can you please have a look at the errors reported by our static analysis?

I'd like, but without deep knowledge of internals I can't guarantee anything 😑

[derrabus]

Give it a try. If you encounter errors that you don't know how to fix, ask us anything. 🙂

[norkunas]

I see a contradiction between psalm and phpstan.
From psalm I get:

ERROR: PossiblyUndefinedArrayOffset
at /orm/src/Internal/Hydration/AbstractHydrator.php:350:22
Possibly undefined array key $newObject['args'] on array{args?: array<array-key, BackedEnum|array<array-key, BackedEnum>|mixed>, class: mixed} (see https://psalm.dev/167)
            $args  = $newObject['args'];


ERROR: PossiblyUndefinedArrayOffset
at /orm/src/Internal/Hydration/AbstractHydrator.php:359:26
Possibly undefined array key $newObject['args'] on array{args?: array<array-key, BackedEnum|array<array-key, BackedEnum>|mixed>, class: mixed} (see https://psalm.dev/167)
                $args  = $newObject['args'];

But if I modify to $newObject['args'] ?? [] then phpstan says:

Error: Offset 'args' on array{class: mixed, args: non-empty-array<int|string, mixed>} on left side of ?? always exists and is not nullable.
Error: Offset 'args' on array{class: mixed, args: non-empty-array<int|string, mixed>} on left side of ?? always exists and is not nullable