feature #21164 [HttpKernel] Added the SessionValueResolver (iltar)

This PR was merged into the 3.3-dev branch.

Discussion
----------

[HttpKernel] Added the SessionValueResolver

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #21159
| License       | MIT
| Doc PR        | (soon)

This feature adds the `SessionValueResolver`. That means that you no longer have to rely on injecting a `SessionInterface` implementation via the constructor or getting this implementation from the `Request`. Regardless of method, it does not know about the `getFlashBag()`.

By adding the `Session` to the action arguments, you can now type-hint against the implementation rather than interface, which contains the `getFlashBag()`, making it accessible rather than using duck-typing.

_It should also feel less like injecting a service into the constructor which has a state or getting a service from the request._

**Old Situation**
```php
class Controller
{
    public function __construct(SessionInterface $session) { /* ... */ }

    public function fooAction(Request $request)
    {
        $this->get('session')->get(...);
        $request->getSession()->get(...);
        $this->session->get(...)

        // duck-typing
        $this->get('session')->getFlashBag();
        $request->getSession()->getFlashBag();
        $this->session->getFlashBag();

        $this->addFlash(...);
    }
}
```

**New Situation** _- The controller shortcut for flashbag could in theory be removed now_
```php
class Controller
{
    public function fooAction(Session $session)
    {
        $session->get(...);
        $session->getFlashBag();
    }
}
```

Commits
-------

b4464dcea1 Added the SessionValueResolver

Author: fabpot

Resources/config/web.xml

@@ -32,6 +32,10 @@
             <tag name="controller.argument_value_resolver" priority="50" />
         </service>
 
+        <service id="argument_resolver.session" class="Symfony\Component\HttpKernel\Controller\ArgumentResolver\SessionValueResolver" public="false">
+            <tag name="controller.argument_value_resolver" priority="50" />
+        </service>
+
         <service id="argument_resolver.default" class="Symfony\Component\HttpKernel\Controller\ArgumentResolver\DefaultValueResolver" public="false">
             <tag name="controller.argument_value_resolver" priority="-100" />
         </service>