[8.x] Model::encryptUsing()

[hivokas]

encrypted casts have been added to Laravel recently. While this feature is extremely useful, sometimes developers may want to use the separate key to encrypt database values. Currently, that's not possible. However, this PR introduces this ability by adding Model::encryptUsing() method that accepts encrypter instance that will be used to perform encryption-related casts.

The most typical use of this would be a similar piece of code in AppServiceProvider or in a separate service provider:

use Illuminate\Database\Eloquent\Model;
use Illuminate\Encryption\Encrypter;

$databaseEncryptionKey = config('database.encryption_key');

$encrypter = new Encrypter($databaseEncryptionKey);

Model::encryptUsing($encrypter);