Add Gotify notification support to ZED

This commit adds the zed_notify_gotify() function and hooks it
into zed_notify(). This will allow ZED to send notifications
to a self-hosted Gotify service, which can be received
on a desktop or mobile device. It is configured with ZED_GOTIFY_URL,
ZED_GOTIFY_APPTOKEN and ZED_GOTIFY_PRIORITY variables in zed.rc.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: gofaster <felix.gofaster@gmail.com>
Closes #15693