feature #47943 [Config][Routing] Nicer config syntax for PSR-4 route loading (derrabus)

This PR was merged into the 6.2 branch.

Discussion
----------

[Config][Routing] Nicer config syntax for PSR-4 route loading

| Q             | A
| ------------- | ---
| Branch?       | 6.2
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Follow-up to #47916
| License       | MIT
| Doc PR        | symfony/symfony-docs#17373 (WIP)

This PR implements an alternative syntax for the PSR-4 route loader introduced in #47916.

```YAML
controllers:
    resource:
        path: ./Controllers
        namespace: App\Controllers
    type: attribute
```

```XML
<routes>
    <import type="attribute">
        <resource path="./Controllers" namespace="App\Psr4Controllers" />
    </import>
</routes>
```

Commits
-------

d7df3be956 Nicer config syntax for PSR-4 route loading

Author: fabpot

Tests/Functional/app/Psr4Routing/routing.yml

@@ -1,4 +1,6 @@
 test_bundle:
     prefix:   /annotated
-    resource: "@TestBundle/Controller"
-    type:     attribute@Symfony\Bundle\FrameworkBundle\Tests\Functional\Bundle\TestBundle\Controller
+    resource:
+        path: "@TestBundle/Controller"
+        namespace: Symfony\Bundle\FrameworkBundle\Tests\Functional\Bundle\TestBundle\Controller
+    type:     attribute