feat: add --format flag to namespace ls command

[SpiffyEight77]

Description

related Issue #3851

Adds --format flag support to nerdctl namespace ls command for custom output formatting.

Test

nerdctl namespace ls

nerdctl namespace ls
NAME       CONTAINERS    IMAGES    VOLUMES    LABELS
default    0             2         0          env=staging,region=us-west,team=frontend
moby       0             0         0

nerdctl namespace ls -q

nerdctl namespace ls -q
default
moby

nerdctl namespace ls -f '{{json .}}'

nerdctl namespace ls -f '{{json .}}'
{"Name":"default","Containers":0,"Images":2,"Volumes":0,"Labels":{"env":"staging","region":"us-west","team":"frontend"}}
{"Name":"moby","Containers":0,"Images":0,"Volumes":0,"Labels":null}

nerdctl namespace ls -f '{{json .}}' -q

nerdctl namespace ls -f '{{json .}}' -q
FATA[0000] format and quiet must not be specified together

[AkihiroSuda]

Thanks