[DX] New service to simplify password encoding

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11299
| License       | MIT
| Doc PR | https://github.com/symfony/symfony-docs/pull/3995

This new service siplifies the way to encode a password. Just get the `security.password_encoder` service and encode the `User` password.

```php
$encoded = $this->container->get('security.password_encoder')
    ->encodePassword($user, $plainPassword);

$user->setPassword($encoded);
```