Unhide --all flag for plugin group get

[marckhouzam]

What this PR does / why we need it

Following the improvements to the output of tanzu plugin group get --all from #526 , this PR makes the --all flag un-hidden.

Which issue(s) this PR fixes

Fixes # N/A

Describe testing done for PR

# Notice the --all flag in the help
$ tz plugin group get  -h
Get the content of the specified plugin-group.  A plugin-group provides a list of plugin name/version combinations which can be installed in one step.  This command allows to see the list of plugins included in the specified group.

Usage:
tanzu plugin group get GROUP_NAME [flags]

Flags:
      --all             include the contextual plugins
  -h, --help            help for get
  -o, --output string   output format (yaml|json|table)

# Notice that shell completion now suggest the --all flag since it no longer hidden 
$ tz plugin group get vmware-tkg/default -<TAB>
--all         -- include the contextual plugins
--help    -h  -- help for get
--output  -o  -- output format (yaml|json|table)

Release note

Make the `--all` flag of `tanzu plugin group get` not hidden.

Additional information

Special notes for your reviewer

[vuil]

lgtm, thanks