JSON output support for zpool get

This commit adds support for zpool get command to output the list of
properties for ZFS Pools and VDEVS in JSON format using '-j' option.
Man page for zpool get is updated to include '-j' option.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #16217