Feature: Pausing without end time should place the SLA into perpetual hold

The following should put timer into perpetual pause, but also should be optimized enough to break the counter loop (Ie. there won't be any more seconds to count in a perpetual pause)

```php
/** @var SLA $sla */
$sla->addPause('2022-07-21 09:00:00');
```

Maybe even create some nice syntax sugar...

```php
/** @var SLA $sla */
$sla->pauseFrom('2022-07-21 09:00:00');
```