FormTypeExtensionInterface related deprecation

[javiereguiluz]

Describe the bug
I was testing a real app with Symfony 4.2 RC1 and got this deprecation reported:

Not implementing the static getExtendedTypes() method in
EasyCorp\Bundle\EasyAdminBundle\Form\Extension\EasyAdminExtension when
implementing the Symfony\Component\Form\FormTypeExtensionInterface is deprecated
since Symfony 4.2. The method will be added to the interface in 5.0.

@xabbuh as you worked on this, I'd like to ask you if it's safe to add the static getExtendedTypes() method even in 1.x branch or if older Symfony versions won't like it. Thanks!

[yceruto]

I think we should have both methods getExtendedTypes() and getExtendedType(), but the new method should have the code now, so the old one calls to the new one keeping BC prior to 4.2 and removing the deprecation warning since 4.2.

I'm thinking in something like this:

public function getExtendedType(): string
{
    return current(self::getExtendedTypes());
}

public static function getExtendedTypes(): iterable
{
    return [FormType::class];
}

[xabbuh]

@yceruto using current() on an iterable may not play well with static code analyser tools, but apart from that your suggestion looks like the way to go