Add (union) discriminator support

[Spea]

With these changes it is now possible to use (union) discriminators. A few examples which are supported might look like this 👇

Examples

Discriminators

#[JMS\Discriminator(field: 'type', map: ['first' => FirstClass::class, 'second' => SecondClass::class])]
abstract class Discriminator
{
}

class FirstClass extends Discriminator
{
    public string $firstProperty;
}

class SecondClass extends Discriminator
{
    public int $secondProperty;
}

Union Discriminators

use JMS\Serializer\Annotation\UnionDiscriminator;

class JmsUnionTyping
{
    #[UnionDiscriminator(field: 'objectType', map: ['comment' => FirstClass::class, 'author' => SecondClass::class])]
    public FirstClass|SecondClass $property;
}

// ...

class PrimitiveUnionTyping
{
    public int|string|false|float|array|null $property;
}

To support this, it is necessary to use the latest changes from the 2.x branch from the metadata-parser (including the changes necessary for union discriminators via liip/metadata-parser#54).
So ideally this change should go into a new 3.x branch where we then release a new major version, as doing a minor release will potentially break peoples implementation since a few things changed in the parse library.

Side note: There is a temporary commit in here which uses the changes from the other PR in the metadata-parser repo. It will be removed/adjusted, once merged and a new version exists.

[dbu]

very cool, thanks!

i will do php 8.5 and symfony 8 support on the 2.x branch, then add a 3.x branch where we should merge your changes.

before we tag the 3.0 release, we should look at the deprecation and do the BC break to cleanup.
apart from that i am not aware of things we have to do, are there?

[dbu]

i tagged 2.6.2 and now created the 3.x branch. i changed the target, but github reports conflicts. i did apply the cs fixer in the 2.x version, so that is probably not needed anymore. and i added to the changelog - i added the 2.x and 3.x headings so that future changes in 2.x do not conflict with 3.x anymore.

can you please rebase this on the 3.x branch and solve the conflicts?

[Spea]

before we tag the 3.0 release, we should look at the deprecation and do the BC break to cleanup. apart from that i am not aware of things we have to do, are there?

I removed the deprecation warning and adjusted the code around it. Other than that I think we are good here, couldn't find any other deprecations.

i tagged 2.6.2 and now created the 3.x branch. i changed the target, but github reports conflicts. i did apply the cs fixer in the 2.x version, so that is probably not needed anymore. and i added to the changelog - i added the 2.x and 3.x headings so that future changes in 2.x do not conflict with 3.x anymore.

can you please rebase this on the 3.x branch and solve the conflicts?

I resolved the conflicts now, thanks for taking care of the new branch and the cs fixes!

[dbu]

thanks! i tweaked the changelog entry a bit and merged on the CLI. thanks a lot for the contribution!

i tagged https://github.com/liip/serializer/releases/tag/3.0.0 now