[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

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);
```

All the `$this->once()` should actually be `$this->any()`. We don't really care if these methods were called, since we're stubbing here (as opposed to mocking).