[11.x] Provide context for NestedRules

[imahmood]

In the forEach closure, $data containing all the validation data, while $context provides the specific data associated with the current iteration.
This helps to add validation rules based on the context of each field being validated.

// Sample Code
$formData = [
    'users' => [
        ['id' => 1, 'name' => 'Foo', 'file' => ''],
        ['id' => 2, 'name' => 'Bar', 'file' => ''],
    ],
];

$validator = Validator::make($formData, [
    'users.*.file' => Rule::forEach(function ($value, $attribute, $data, $context) {
        dump($data);
        // Output:
        // [
        //   'users.0.id' => '1',
        //   'users.0.name' => 'Foo',
        //   'users.0.file' => '',
        //   'users.1.id' => '2',
        //   'users.1.name' => 'Bar',
        //   'users.1.file' => '',
        // ]

        dump($context);
        // Output:
        // [
        //   'id' => '1',
        //   'name' => 'Foo',
        //   'file' => '',
        // ]

        return [
            $this->userHasAvatar($context['id']) ? 'nullable' : 'required'
        ];
    }),
]);