Add ShouldBeEncrypted attribute

[willrowe]

This adds a ShouldBeEncrypted attribute that can be used instead of needing to define an encrypted method:

// before
use Spatie\LaravelSettings\Settings;

class EncryptedSettings extends Settings
{
    public string $string;

    public static function encrypted(): array
    {
        return ['string'];
    }
}

// after
use Spatie\LaravelSettings\Attributes\ShouldBeEncrypted;
use Spatie\LaravelSettings\Settings;

class EncryptedSettings extends Settings
{
    #[ShouldBeEncrypted()]
    public string $string;
}

The encrypted method still works the same, any properties that have the ShouldBeEncrypted attribute will simply be merged into it.