…cked for uniqueness (wkania)

This PR was squashed before being merged into the 6.1 branch.

Discussion
----------

[Validator] Define which collection keys should be checked for uniqueness

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #9888
| License       | MIT
| Doc PR        | symfony/symfony-docs#16713

Currently, the validator checks each element of the collection as a whole. We already have a custom normalizer (which is great), but it would be nice to be able to check for uniqueness certain [collection](https://symfony.com/doc/current/reference/constraints/Collection.html) keys.

For example, some fields in the collection element can be identifiers. They should be unique within the collection, even when the rest of the element data are different.

Current state:
- validates that all the elements of the given collection are unique

New state:
- preserve the current state,
- all old tests pass (no changes in them),
- no breaking changes,
- define which collection fields should be checked for uniqueness (optional),
- fields are optional in each element of the collection. Use [collection constraints](https://symfony.com/doc/current/reference/constraints/Collection.html) if they are required

Examples:

1. Basic example. Each translation of the same resource must be in a different language.
```php
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @Assert\Count(min=1),
 * @Assert\Unique(fields={"language"}),
 * @Assert\Collection(
 *         fields = {
 *             "language" = {
 *                  @Assert\NotBlank,
 *                  @Assert\Length(min = 2, max = 2),
 *                  @Assert\Language
 *             },
 *             "title" = {
 *                  @Assert\NotBlank,
 *                  @Assert\Length(max = 255)
 *             },
 *             "description" = {
 *                  @Assert\NotBlank,
 *                  @Assert\Length(max = 255)
 *             }
 *         }
 * )
 */
public array $translations = [];
```
2. An example where Optional is recognizable. Items with the id are changed and without are new.
```php
use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\Validator\Constraints\Optional;

/**
 * @Assert\Unique(fields={"id"}),
 * @Assert\Collection(
 *         fields = {
 *             "id" = @Assert\Optional({
 *                  @Assert\Uuid
 *             }),
 *             "name" = {
 *                  @Assert\NotBlank,
 *                  @Assert\Length(max = 255)
 *             }
 *         }
 * )
 */
public array $items = [];
```
3. An example with composite uniqueness
```php
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @Assert\Unique(fields={"latitude", "longitude"}),
 * @Assert\Collection(
 *         fields = {
 *             "latitude" = {
 *                  @Assert\NotBlank
 *             },
 *             "longitude" = {
 *                  @Assert\NotBlank
 *             },
 *             "poi" = {
 *                  @Assert\Length(max = 255)
 *             }
 *         }
 * )
 */
public array $coordinates = [];
```

Commits
-------

0e8f4ce [Validator] Define which collection keys should be checked for uniqueness

…ys (Brajk19)

This PR was squashed before being merged into the 6.4 branch.

Discussion
----------

[Validator] UniqueValidator - normalize before reducing keys

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        |
| License       | MIT

In #42403 checking for uniqueness of certain collection keys was enabled. Method `UniqueValidator::reduceElementKeys` removes all keys which are not specified.
Problem is that this happens before normalization, which in my opinion is not great because that method accepts array argument and if i have some object (DTO), TypeError will be thrown.

Example:

```php
class ParentDTO
{
    /**
     * `@var` ChildDTO[]
     */
    #[Assert\Unique(
        normalizer: [ChildDTO::class, 'normalize']
        fields: 'id'
    )]
    public array $children;
}
```

```php
class ChildDTO
{
    public string $id;
    public string $name;

    public static function normalize(self $obj): array
    {
        return [
            'id' => $obj->id,
            'name' => $obj->name
        ];
    }
}
```

Because normalization will happen after `reduceElementKeys` this will be thrown:
`TypeError: Symfony\Component\Validator\Constraints\UniqueValidator::reduceElementKeys(): Argument #2 ($element) must be of type array, ...\ChildDTO given, called in .../UniqueValidator.php on line 48`

If `$element = $normalizer($element);` is executed before `reduceElementKeys` it would enable using Assert\Unique with array of objects when correctly normalized

Commits
-------

77df90b [Validator] UniqueValidator - normalize before reducing keys

…ys (Brajk19)

This PR was squashed before being merged into the 6.4 branch.

Discussion
----------

[Validator] UniqueValidator - normalize before reducing keys

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        |
| License       | MIT

In symfony/symfony#42403 checking for uniqueness of certain collection keys was enabled. Method `UniqueValidator::reduceElementKeys` removes all keys which are not specified.
Problem is that this happens before normalization, which in my opinion is not great because that method accepts array argument and if i have some object (DTO), TypeError will be thrown.

Example:

```php
class ParentDTO
{
    /**
     * `@var` ChildDTO[]
     */
    #[Assert\Unique(
        normalizer: [ChildDTO::class, 'normalize']
        fields: 'id'
    )]
    public array $children;
}
```

```php
class ChildDTO
{
    public string $id;
    public string $name;

    public static function normalize(self $obj): array
    {
        return [
            'id' => $obj->id,
            'name' => $obj->name
        ];
    }
}
```

Because normalization will happen after `reduceElementKeys` this will be thrown:
`TypeError: Symfony\Component\Validator\Constraints\UniqueValidator::reduceElementKeys(): Argument #2 ($element) must be of type array, ...\ChildDTO given, called in .../UniqueValidator.php on line 48`

If `$element = $normalizer($element);` is executed before `reduceElementKeys` it would enable using Assert\Unique with array of objects when correctly normalized

Commits
-------

77df90b959 [Validator] UniqueValidator - normalize before reducing keys