[11.x] Add RequiredIfDeclined validation rule

[timmydhooghe]

Description

This PR adds a required_if_declined rule. The field under validation must be present and not empty if the other field field is declined (it uses the existing validateDeclined rule).

☑️ A test is added.
I'll create a PR for the documentation if this gets merged

Use case

Entering an expense:

• if the cost is recurring (subscription), we need to know the frequency (weekly, monthy, yearly)
  • required_if_accepted can be used to make the frequency mandatory
• if the cost is one-off (restaurant receipt), we need the receipt date.
  • required_if_declined can be used to make the receipt date mandatory

Recurring:

One-off:

---

Validator::make([
    'is_recurring' => '0',
    'receipt_date' => '2024-04-11',
], [
    'is_recurring' => 'required|boolean',
    'receipt_date' => 'required_if_declined:is_recurring',
]);

[timmydhooghe]

@taylorotwell There's one test failing:

1) Illuminate\Tests\Encryption\EncrypterTest::testRawStringEncryptionWithPreviousKeys

Failed asserting that two strings are identical.

I only added a test in tests/Validation/ValidationValidatorTest.php, so I'm a bit confused.