[PHP 8.1] Fibers

Problem/Motivation

PHP 8.1 adds Fibers, opening this meta to group together potential use cases.

https://php.watch/versions/8.1/fibers

https://www.php.net/releases/8.1/en.php

Steps to reproduce

Proposed resolution

Queues: #1189464: Add an 'instant' queue runner with Fibers
Cron: #3257727: Run automated cron in a Fiber
Rebuilds: #3257725: Use Fibers to offload discovery builds

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet