[8.x] Allow to fillJsonAttribute with encrypted fields

[dkulyk]

This PR allow to use fillJsonAttribute with encrypted fields. (same as #36056 for 9.x)

public $casts = [
        'secret_json' => 'encrypted:json',
        'secret' => 'encrypted',
];

$model->setAttribute('secret_json->key', 'value');
$model->fill([
  'secret->id' => 'id',
  'secret->key' => 'key'
]);

[taylorotwell]

Breaking changes?