[2.5][Form] solved dependency to ValidatorInterface, fix #11036

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | #11036, #11345
| License       | MIT
| Doc PR        | 

Since Symfony 2.5

The problem was that the form component has a hardcoded depencency to the deprecated validator component (api Version 2.4)
The pull request fixes the dependency to the validator component and supports now both implementations, apiVersion 2.5 and apiVersion 2.4 of the validator component.

@Symfony Core Members
please review the changes https://github.com/sebastianblum/symfony/blob/0a1e9c208f8730219bebf89f6696b246a0c88da7/src/Symfony/Component/Form/Extension/Validator/ValidatorExtension.php
I'm not sure if it was the right solution

Also IIRC proper phpdoc here should be:
```php
use Symfony\Component\Validator\Mapping\ClassMetadata;

/** @var $metadata ClassMetadata */