[ML] Calendars : Allow calendar to apply to all jobs

[tazbert]

Allow a ML calendar to apply to all ML jobs defined in the system.

Currently, calendars have lists of jobs and/or groups that the calendar applies to. There are some situations where the calendar should apply to all ML jobs. There is no mechanism for this. It is possible to put all jobs into a default group and attach that to the calendar, but there is an administrator overhead to making sure all new jobs are put in this group.

Some brief discussion in the forum: https://discuss.elastic.co/t/calendar-events-and-machine-learning-jobs/191678

[elasticmachine]

Pinging @elastic/ml-core