[WIP] [Serializer] Serialization groups support

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | not yet

This PR is a first attempt adding serialization groups to the `Serializer` component. Btw, it also add supports of ignored attributes for denormalization (only normalization is currently supported).

Groups support is totally optional and is not enabled by default (in that case, the `Serializer` will have the current behavior). No BC spotted for now.

To use it:
```php
use Symfony\Component\Serializer\Annotation\Groups;

use Symfony\Component\Serializer\Serializer;
use Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer;
use Symfony\Component\Serializer\Mapping\Loader\AnnotationLoader;
use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;

class MyObj
{
    /**
     * @Groups({"group1", "group2")
     */
    public $foo;
    /**
     * @Groups({"group3")
     */
    public $bar;
}

$obj = new MyObj();
$obj->foo = 'foo';
$obj->bar = 'bar';

$classMetadataFactory = new ClassMetadataFactory(new AnnotationLoader(new AnnotationReader()));
$normalizer = new PropertyNormalizer($classMetadataFactory);

$serializer = new Serializer([$normalizer]);
$data = $serializer->normalize($obj, null, ['groups' => ['group1']]);
// $data = ['foo' => 'foo'];

$obj2 = $serializer->denormalize(['foo' => 'foo', 'bar' => 'bar'], 'MyObj', null, ['groups' => ['group1', 'group3']);
// $obj2 = MyObj(foo: 'foo', bar: 'bar')
```

Some work still need to be done:
- [ ] Add XML mapping
- [ ] Add YAML mapping
- [ ] Refactor tests

The `ClassMetadata` code is largely inspired from the code of the `Validator` component. Duplicated code in `PropertyNormalizer` and `GetSetMethodNormalizer` has been moved in a new `AbstractNormalizer` class.
You're right!
I'll change the exception message.