feature: watchdog workflow: automated issues when things fail

There could be a failure watchdog in addtition to #96:

on a cron workflow , use a TUF client to fetch the metadata from the published location
file an issue if refresh actually falls
file an issue if online roles are within signing-period: in other words there should have been a version bump already: note that this requires some nuance as online signing happens in cron by default every 6 hours -- so currently we may be 6 hours into the signing period before the signing happens.

jku / repository-playground