Allow for being "ping"ed to monitor local CRON jobs

[yarikoptic]

Ultimate goal is to replace https://healthchecks.io/ which is used simply by having at the end of the cron job to && curl -fsS -m 10 --retry 5 -o /dev/null https://hc-ping.com/61dfdedd-SENSORED-UUID and thus service recording that there was successful (due to && condition) that cron job execution on my server.

I am not yet sure on what could be the easiest way to do smth like that via upptime, but I guess could be providing/updating some ref directly on github repo, thus would need some github token or ssh setup etc. And then upptime CI job collecting those. We have somewhat loosely similar setup to "harvest" client build jobs in https://github.com/datalad/git-annex which populates job/result branches in https://github.com/datalad/git-annex-ci-client-jobs which are then picked up by CI.