This PR builds on #61528

I propose to add a #[ValidationFor] attribute that allows adding validation constraints 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-validation

At the moment, the only way to achieve this is by declaring the new constraints in the (hardcoded) config/validation/ folder, using either xml or yaml. No attributes.

With this PR, one will be able to define those extra constraints 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 constraints; target = the existing class to add constraints to.)

#[ValidationFor(TargetClass::class)]
class SourceClass
{
    #[Assert\NotBlank(groups: ['my_app'])]
    #[Assert\Length(min: 3, groups: ['my_app'])]
    public string $name = '';

    #[Assert\Email(groups: ['my_app'])]
    public string $email = '';

    #[Assert\Range(min: 18, groups: ['my_app'])]
    public int $age = 0;
}

Here are the basics of how this works:

1. During container compilation, classes marked with #[ValidationFor(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 validator is configured to map the target to its source classes.
3. At runtime, when loading validation metadata for the target, attributes (constraints, callbacks, group providers) are read from both the target and its mapped source classes and applied accordingly.