Add `wait()` method to Task class

**Description**

Add `wait(int $timeoutInMs, int $intervalInMs)` method to Task class.


**Basic example**

So instead of:

```php
$task = $this->client->createIndex('new-index', ['primaryKey' => 'objectID']);
$task = $this->index->waitForTask($task->getTaskUid());
```

We could write:

```php
$task = $this->client->createIndex('new-index', ['primaryKey' => 'objectID'])->wait();
```
