Fix: compatibility with all (>= 3.0) rufus-scheduler versions

[kares]

The work here is meant to get the plugin working with up-to-date rufus-scheduler versions.

ATM, LS 7.x is stuck with gem version 3.0.9 (due plugins pinning ~> 3.0.9), see elastic/logstash#12931

The previous rufus-scheduler < 3.5 lock (got tightened in version 3.2.2 to ~> 3.0.9), was due Rufus moving the Rufus::Scheduler::CronLine class into a standalone gem (Fugit::Cron) with a slightly changed API (removed #days method). That feature was only used for logging and is now replaced by simply logging the calculated frequency of the cron expression.

Also, part of the PR our custom Scheduler sub-class was reviewed for re-use with a new API draft to use across plugins (as we plan to extract the Scheduler mixin bits into its own gem), the idea being using a factory method to provide everything in one step e.g.

      @scheduler = ...::Scheduler.
          start_cron_scheduler(schedule, thread_name: "[#{id}]<...") { do_periodic_work(queue) }
      @scheduler.join

Most plugins will only need ^^^ this, the JDBC plugin still rely on some Rufus::Scheduler api (not worth to push the scheduler externalization that much further atm).

NOTE: we should consider not using any 3.1 - 3.5 version which had stopped using tzinfo gem for a self invented Rufus::ZoTime class, tzinfo is (transitively) used again in version >= 3.6 through an added gem.

[andsel]

LGTM!

A nice work in moving the Scheduler code in a common mixin