Add Ntfy notification support to ZED

This commit adds the zed_notify_ntfy() function and hooks it
into zed_notify(). This will allow ZED to send notifications
to ntfy.sh or a self-hosted Ntfy service, which can be received
on a desktop or mobile device. It is configured with ZED_NTFY_TOPIC,
ZED_NTFY_URL, and ZED_NTFY_ACCESS_TOKEN variables in zed.rc.

Reviewed-by: @classabbyamp
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Dex Wood <slash2314@gmail.com>
Closes #15584