Add trim_finish notify script for ZED

Allow users to configure notifications when TRIM operations are
completed on pools. Unlike resilver_finish and scrub_finish,
the trim_finish event is generated for each vdev in the pool
which was trimmed, so the script will generate a notification
for each one.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kevin P. Fleming <kevin@km6g.us>
Closes #10491