CLI feature: Show which jobs are currently utilizing a8c_cron_control_concurrent_event_whitelist

A WP-CLI command like `wp cron-control show-allowed-concurrent` would be handy to show which jobs are currently utilizing the filter. It would be helpful to aggregate and display that data when debugging stale queues.

Currently to find out, either need to search codebase for `a8c_cron_control_concurrent_event_whitelist` (which can get cumbersome on sites that use multiple repos), or run the filter to get the output with `wp shell`:

```
# wp shell
wp> apply_filters( 'a8c_cron_control_concurrent_event_whitelist', [] );
=> array(2) {
...
```