feature #11306 [DX] New service to simplify password encoding (aferrandini)

This PR was merged into the 2.6-dev branch.

Discussion
----------

[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 | symfony/symfony-docs#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);
```

Commits
-------

7bc190a New service to simplify password encoding

Author: fabpot

Resources/config/security.xml

@@ -10,6 +10,7 @@
         <parameter key="security.user_checker.class">Symfony\Component\Security\Core\User\UserChecker</parameter>
 
         <parameter key="security.encoder_factory.generic.class">Symfony\Component\Security\Core\Encoder\EncoderFactory</parameter>
+        <parameter key="security.user_password_encoder.generic.class">Symfony\Component\Security\Core\Encoder\UserPasswordEncoder</parameter>
         <parameter key="security.encoder.digest.class">Symfony\Component\Security\Core\Encoder\MessageDigestPasswordEncoder</parameter>
         <parameter key="security.encoder.plain.class">Symfony\Component\Security\Core\Encoder\PlaintextPasswordEncoder</parameter>
         <parameter key="security.encoder.pbkdf2.class">Symfony\Component\Security\Core\Encoder\Pbkdf2PasswordEncoder</parameter>
@@ -80,6 +81,12 @@
 
         <service id="security.encoder_factory" alias="security.encoder_factory.generic"></service>
 
+        <service id="security.user_password_encoder.generic" class="%security.user_password_encoder.generic.class%" public="false">
+            <argument type="service" id="security.encoder_factory"></argument>
+        </service>
+
+        <service id="security.password_encoder" alias="security.user_password_encoder.generic"></service>
+
         <service id="security.user_checker" class="%security.user_checker.class%" public="false" />
 
         <service id="security.expression_language" class="%security.expression_language.class%" public="false" />