[5.3] Add $encrypts property that encrypts model attributes

[djtarazona]

This PR is similar to #16443 but the design is a bit different in that you specify attributes to be encrypted in the $encrypts property. This allows attributes to be encrypted and casted.

Use case:

// User.php
class User extends Model
{
    protected $casts = [
        'github_oauth_token' => 'array',
    ];

    protected $encrypts = [
        'github_oauth_token',
    ];
}

// Controller.php (or wherever)
$user = new User();

$user->github_oauth_token = [
    'access_token' => 'abc123...',
    'refresh_token' => 'def456...',
    'expires_at' => 1479423404,
];

$user->getAttributes()['github_oauth_token']; // gibberish (as it is not decrypted) — stored in DB encrypted

This may be considered a breaking change because of the new $encrypts property? If so, I'd be happy to send this to 5.4.

[djtarazona]

Shout out to @alnutile for his great work on #16443, I just need something a bit more flexible that allows me to cast and encrypt attributes. :)

[taylorotwell]

I would rather have support for multiple casts.

[djtarazona]

@taylorotwell When supporting multiple casts, you'd have to be careful to make sure that encryption is run after all other casts when setting and decryption before casts when getting.

Also, when would you need multiple casts other than to use encryption with something else? I find it unlikely that you'd, for example, cast to a string and then to an int, etc.