feat: better debug info for PHP messages and repeated fields (#12718)

addresses https://github.com/protocolbuffers/protobuf/issues/12714 by dumping more concise debug info for protobuf messages and repeated fields via the `serializeToJsonString` function. Additionally, message types which serialize into something other than an array (e.g. `Google\Protobuf\Value`, `Google\Protobuf\Timestamp`, etc) are handled in a special way to make their output consistent with other messages.

```php
$m = new Google\Protobuf\DoubleValue();
$m->setValue(1.5);
var_dump($m);
```
will output
```
object(Google\Protobuf\DoubleValue)#12 (1) {
  ["value"]=>
  float(1.5)
}
```

Closes #12718

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/12718 from bshaffer:php-add-debuginfo c40a6f91de08a39d9f66c7c5a5463893edcbedf8
PiperOrigin-RevId: 574115431