[7.x] Allow developers to specify accepted keys in array rule

[sileence]

I'm trying to come up with a solution for the problem @mnabialek mentioned here #32379:

---

I'm sending such data in test:

$response = $this->post('/validate', [
    'users' => [
        [
            'name' => 'Person 1',
            'email' => 'sample@example.com',
            'is_admin' => true,
        ]
    ],
    'anything' => 'else',
]);

And my controller method looks like this:

$validated = $this->validate(request(), [
    'users' => ['required', 'array'],
    'users.*' => ['required', 'array'],
    'users.*.name' => ['required', 'string'],
    'users.*.email' => ['required', 'email'],
]);

dd($validated); returns:

array:1 [
  "users" => array:1 [
    0 => array:3 [
      "name" => "Person 1"
      "email" => "sample@example.com"
      "is_admin" => true
    ]
  ]
]

Although is_admin flag shouldn't be there.

---

In cases like this the validation could be changed slightly, specifying the keys that are accepted in any given array:

$validated = $this->validate(request(), [
    'users' => ['required', 'array'],
    'users.*' => ['required', 'array:name,email'],
    'users.*.name' => ['required', 'string'],
    'users.*.email' => ['required', 'email'],
]);

In this case both keys happened to be required and therefore they have their own validated rules, but that is not always going to be the case.

I was also trying to limit the amount of data in the validated array when more nested rules are specified but that's turning out to be a bit more difficult and also a breaking change, so I'm sending this rule option in the meantime as a way to whitelist the accepted keys in associative arrays.