…listed in the input (Christian Kolb)

This PR was merged into the 6.3 branch.

Discussion
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[Serializer] Add flag to require all properties to be listed in the input

| Q             | A
| ------------- | ---
| Branch?       | 6.3
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | Fix #49504
| License       | MIT
| Doc PR        | symfony/symfony-docs#17979

The PR #40522 introduced a fallback for nullable properties to be set to `null` when they aren't provided as parameters.

A drawback of that approach is that it easier for bugs to appear through typos or renamings of those properties. I think the current implementation makes perfect sense as a default. Therefore, this PR introduces a new context flag that prevents that fallback behaviour. This way nothing changes for existing systems, but for people wanting more control, it's possible to set a flag.

### Example

```php
final class Product
{
    public function __construct(
        public string $name,
        public ?int $costsInCent,
    ) {
    }
}

// This works and results in $costsInCent as null
$product = $this->serializer->deserialize(
    '{"name": "foo"}',
    Product::class,
    JsonEncoder::FORMAT,
);

// When using the flag, only the following JSON is valid
$product = $this->serializer->deserialize(
    '{"name": "foo", "costsInCent": null}',
    Product::class,
    JsonEncoder::FORMAT,
    [
        AbstractNormalizer::PREVENT_NULLABLE_FALLBACK => true,
    ],
);

// This would result in an error due to missing parameters
$product = $this->serializer->deserialize(
    '{"name": "foo"}',
    Product::class,
    JsonEncoder::FORMAT,
    [
        AbstractNormalizer::PREVENT_NULLABLE_FALLBACK => true,
    ],
);
```

Commits
-------

d62410a [Serializer] Add flag to require all properties to be listed in the input