…re new serialization attributes for a class (nicolas-grekas)This PR was merged into the 7.4 branch.Discussion----------[Serializer] Add `#[ExtendsSerializationFor]` to declare new serialization attributes for a class| Q             | A| ------------- | ---| Branch?       | 7.4| Bug fix?      | no| New feature?  | yes| Deprecations? | no| Issues        | -| License       | MITThis PR builds on#61532It's a sibling of#61545I propose to add a `#[ExtendsSerializationFor]` attribute that allows adding serialization attributes to another class.This is typically needed for third party classes. For context, Sylius has a nice doc about this:https://docs.sylius.com/the-customization-guide/customizing-serialization-of-apiAt the moment, the only way to achieve this is by declaring the new attributes in the (hardcoded) `config/serialization/` folder, using either xml or yaml. No attributes.With this PR, one will be able to define those extra serialization attributes using PHP attributes, set on classes that'd mirror the properties/getters of the targeted class. The compiler pass will ensure that all properties/getters declared in these source classes also exist in the target class. (source = the app's class that declares the new serialization attributes; target = the existing class to add serialization attributes to.)```php#[ExtendsSerializationFor(TargetClass::class)]abstract class SourceClass{    #[Groups(['my_app'])]    #[SerializedName('fullName')]    public string $name = '';    #[Groups(['my_app'])]    public string $email = '';    #[Groups(['my_app'])]    #[MaxDepth(2)]    public Category $category;}```(I made the class abstract because it's not supposed to be instantiated - but it's not mandatory.)Here are the basics of how this works:1. During container compilation, classes marked with `#[ExtendsSerializationFor(Target::class)]` are collected and validated: the container checks that members declared on the source exist on the target. If not, a `MappingException` is thrown.2. The serializer is configured to map the target to its source classes.3. At runtime, when loading serialization metadata for the target, attributes (groups, serialized names, max depth, etc.) are read from both the target and its mapped source classes and applied accordingly.Commits-------386f949 [Serializer] Add `#[ExtendsSerializationFor]` to declare new serialization attributes for a class