Add zfs_prepare_disk script for disk firmware install

Have libzfs call a special zfs_prepare_disk script before a disk is
included into the pool. The user can edit this script to add things
like a disk firmware update or a disk health check. Use of the script
is totally optional. See the zfs_prepare_disk manpage for full details.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #15243