Symfony version(s) affected: 4.2+

Description
When using a parameter as the class given to a service Definition, the FormPass will trigger a deprecation warning or throw an Exception for not being able to ensure method getExtendedTypes exists.

How to reproduce

Create a form type extension:

// src/App/Form/Extension/FooFormExtension
namespace App\Form\Extension;

use Symfony\Component\Form\AbstractTypeExtension;
use Symfony\Component\Form\Extension\Core\Type\FormType;

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

Set up the configuration:

# config/services.yaml
parameters:
    foo_form_service_class: 'App\Form\Extension\FooFormExtension'
services:
    foo_form_service:
        class: '%foo_form_service_class%'
        tags:
            - { name: form.type_extension, extended_type: Symfony\Component\Form\Extension\Core\Type\FormType }

Now, when running your app, you'lle get this deprecation warning:

Not implementing the static getExtendedTypes() method in %foo_form_service_class% when implementing the Symfony\Component\Form\FormTypeExtensionInterface is deprecated since Symfony 4.2. The method will be added to the interface in 5.0.

If you set up the service without a backwards-compatible definition such as:

# config/services.yaml
parameters:
    foo_form_service_class: 'App\Form\Extension\FooFormExtension'
services:
    foo_form_service:
        class: '%foo_form_service_class%'
        tags: ['form.type_extension']

Then an exception will be thrown:

(1/1) InvalidArgumentException "form.type_extension" tagged services have to implement the static getExtendedTypes() method. The class for service "foo_form_service" does not implement it.

Possible Solution
Other than trying to resolve the class as a parameter in Symfony\Component\Form\DependencyInjection\FormPass, it could be possible to deprecate using parameters to provide the class of a service altogether.

Additional context
This is all happening in Symfony\Component\Form\DependencyInjection\FormPass::processFormTypeExtensions().

I've noticed this while using LexikFormFilterBundle.