[Stopwatch] added __toString on StopwatchEvent

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12152 
| License       | MIT
| Doc PR        | no

This allow to just print the events in a log or something.

```php
$stopwatch = new Stopwatch();
$event = $stopwatch->start('foo', 'cat');

echo $event; // cat: 0.00 MiB - 0 ms
```