Support UI for async with Batch API, add "delay" option for progress bar.

Problem/Motivation

The Batch API provides a progress bar that could potentially be used not only for a queue of actions but also to indicate a long asynchronous operation on the backend by checking its status.

Currently, this can be achieved using `sleep()` in the batch action callback. However, this is not an ideal solution for performance. While a sleeping PHP process does not use CPU resources, it still maintains an active connection and occupies both the web server worker and the PHP-FPM worker for the entire duration.

The best approach would be to enable the UI to send short requests with a configurable delay. While `core/drupal.progress` supports a delay, `core/drupal.batch` always sets it to 10. Additionally, some jobs require executing only one operation per request.

Some abstract example:

  // In form submit:
  shell_exec("$long_command > $filename 2>/dev/null &");

  // Send 1 request per second to BE.
  $batch_builder = (new BatchBuilder())
      ->setTitle($this->t('Please wait'))
      ->setDelay(1000);

  $batch_builder->addOperation('my_module_check_exec_status', [$filename]);
  $batch_builder->addOperation('my_module_process_file', [$filename]);

  batch_set($batch_builder->toArray());

function my_module_check_exec_status($filename, array &$context) {
    if (!isset($context['sandbox']['finished'])) {
      $context['sandbox']['finished'] = 0;
    }
    $context['sandbox']['finished']++;

    if (file_exists($filename)) {
      $context['finished'] = 1;
    } else {
      $context['finished'] = min($context['sandbox']['finished']/100, .99);
    }
}

Proposed resolution

Add a "delay" property to the batch definition, which would be passed to the progress bar and allow only one operation per request. This appears to be a straightforward change — see the example provided in the MR.

Remaining tasks

- Tests
- Non-progressive batch exception or `sleep()` implementation (needs discussion)

User interface changes

No

API changes

`batch_set`:

 *   - delay: A number of milliseconds between operations. Sets browser's
 *     progress bar delay, only one operation would be performed per request
 *     if positive value provided.

`BatchBuilder`

  /**
   * Sets delay in milliseconds between operations for a progressive batch.
   *
   * Doesn't affect non-progressive batch.
   *
   * @param int $delay
   *   (optional) Delay in milliseconds between operations. Sets browser's
   *   progress bar delay, only one operation would be performed per request
   *   if positive value provided. Resets value to default (0) if the param
   *   is not provided.
   *
   * @return $this
   */
  public function setDelay($delay = 0) {
    $this->delay = $delay;
    return $this;
  }