nvmeofgw: introduce nvme-gw show-all

Introduce a new read-only command, nvme-gw show-all, which
iterates over all configured pools and groups and runs
nvme-gw show for each, simplifying gateway inspection.

fixes https://tracker.ceph.com/issues/74854
Signed-off-by: Leonid Chernin <leonidc@il.ibm.com>