Allow triggering via cookie and env var as well as param similar to xhprof / tideways

[brendanheywood]

https://github.com/moodle/moodle/blob/main/lib/xhprof/xhprof_moodle.php#L106C23-L109

https://github.com/moodle/moodle/blob/main/lib/xhprof/xhprof_moodle.php#L189-L194