Fix EZP-23094: Make SiteAccess aware semantic configuration reusable and extensible

https://jira.ez.no/browse/EZP-23094

This patch makes it easy to implement a *SiteAccess aware* semantic configuration and its mapping to internal config for any eZ bundle developer.

Concrete implementation: https://github.com/ezsystems/CommentsBundle/pull/9
[Complete documentation of the feature is here](https://github.com/ezsystems/ezpublish-kernel/pull/931/files?short_path=49e30ff#diff-49e30ff8b9fcc559e5b49b9e07070047).

## Semantic configuration parsing
An abstract `Configuration` class has been added, simplifying the way to add a SiteAccess settings tree like the following:

```yaml
acme_demo:
    system:
        my_siteaccess:
            hello: "world"
            foo_setting:
                an_integer: 456
                enabled: true

        my_siteaccess_group:
            hello: "universe"
            foo_setting:
                foo: "bar"
                some: "thing"
                an_integer: 123
                enabled: false
```

All you have to do is to extend it and use `$this->generateScopeBaseNode()`:

```php
namespace Acme\DemoBundle\DependencyInjection;

use eZ\Bundle\EzPublishCoreBundle\DependencyInjection\Configuration\SiteAccessAware\Configuration as ScopeConfiguration;
use Symfony\Component\Config\Definition\Builder\NodeBuilder;
use Symfony\Component\Config\Definition\Builder\TreeBuilder;

class Configuration extends ScopeConfiguration
{
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root( 'acme_demo' );

        // $systemNode will then be the root of siteaccess aware settings.
        $systemNode = $this->generateScopeBaseNode( $rootNode );
        $systemNode
            ->scalarNode( 'hello' )->isRequired()->end()
            ->arrayNode( 'foo_setting' )
                ->children()
                    ->scalarNode( "foo" )->end()
                    ->scalarNode( "some" )->end()
                    ->integerNode( "an_integer" )->end()
                    ->booleanNode( "enabled" )->end()
                ->end()
            ->end();

        return $treeBuilder;
    }
```

## Mapping to internal settings
Semantic configuration must always be *mapped* to internal *key/value* settings within the ServiceContainer. Usually this is done in the DIC extension.
For SiteAccess aware settings, new `ConfigurationProcessor` and `Contextualizer` classes have been introduced to ease the process.

```php
namespace Acme\DemoBundle\DependencyInjection;

use eZ\Bundle\EzPublishCoreBundle\DependencyInjection\Configuration\SiteAccessAware\ConfigurationProcessor;
use eZ\Bundle\EzPublishCoreBundle\DependencyInjection\Configuration\SiteAccessAware\ContextualizerInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\Config\FileLocator;
use Symfony\Component\HttpKernel\DependencyInjection\Extension;
use Symfony\Component\DependencyInjection\Loader;

/**
 * This is the class that loads and manages your bundle configuration
 *
 * To learn more see {@link http://symfony.com/doc/current/cookbook/bundles/extension.html}
 */
class AcmeDemoExtension extends Extension
{
    public function load( array $configs, ContainerBuilder $container )
    {
        $configuration = $this->getConfiguration( $configs, $container );
        $config = $this->processConfiguration( $configuration, $configs );

        $loader = new Loader\YamlFileLoader( $container, new FileLocator( __DIR__.'/../Resources/config' ) );

        $processor = new ConfigurationProcessor( $container, 'acme_demo' );
        $processor->mapConfig(
            $config,
            // Any kind of callable can be used here
            function ( $scopeSettings, $currentScope, ContextualizerInterface $contextualizer )
            {
                // Will map "hello" setting to "acme_demo.<$currentScope>.hello" container parameter
                // It will then be possible to retrieve this parameter through ConfigResolver in the application code:
                // $helloSetting = $configResolver->getParameter( 'hello', 'acme_demo' );
                $contextualizer->setContextualParameter( 'hello', $currentScope, $scopeSettings['hello'] );
            }
        );

        // Now map "foo_setting" and ensure keys defined for "my_siteaccess" overrides the one for "my_siteaccess_group"
        // It is done outside the closure as it is needed only once.
        $contextualizer = $processor->getContextualizer();
        $contextualizer->mapConfigArray( 'foo_setting', $config );
    }
}
```

### Dedicated mapper object
Instead of passing a callable to `$processor->mapConfig()`, an instance of `eZ\Bundle\EzPublishCoreBundle\DependencyInjection\Configuration\SiteAccessAware\ConfigurationMapper` can be passed, for example if you have a lot of configuration to map and don't want to *pollute* your DIC extension class.

## TODO
* [x] Move phpdoc to interfaces
* [x] Add a clear specification doc
* [x] Tests!
Not specified: http://www.phpdoc.org/docs/latest/references/phpdoc/tags/param.html

So I'd conservatively keep it as is :-)