Adding slack notifier

Allow ZED notification via slack incoming webhook.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Signed-off-by: Ben McGough <bmcgough@fredhutch.org>
Closes #9076
Closes #9350