Add the "plugin group get" command

[marckhouzam]

This PR is on top of #270 to first provide plugin group version support. Only the top-most commit is new

What this PR does / why we need it

This PR adds a command to see the list of plugins part of a plugin group version (finally!):

$ tz plugin group get vmware-tkg/default:v2.1.1
Plugins in Group:  vmware-tkg/default:v2.1.1
  NAME                TARGET      LATEST
  cluster             kubernetes  v0.28.1
  feature             kubernetes  v0.28.1
  isolated-cluster    global      v0.28.1
  kubernetes-release  kubernetes  v0.28.1
  management-cluster  kubernetes  v0.28.1
  package             kubernetes  v0.28.1
  pinniped-auth       global      v0.28.1
  secret              kubernetes  v0.28.1
  telemetry           kubernetes  v0.28.1

The user can omit the version to get the content of the latest version of a group: tz plugin group get vmware-tkg/default

Which issue(s) this PR fixes

Fixes # N/A

Describe testing done for PR

# Initialize stuff
$ rm ~/.cache/tanzu/plugin_inventory
$ rm ~/.config/tanzu/config*
$ \rm -rf hack/central-repo/registry-content
$ make start-test-central-repo
[...]
$ tz plugin source update default -u localhost:9876/tanzu-cli/plugins/central:small
[ok] updated discovery source default
$ tz config set env.TANZU_CLI_PLUGIN_DISCOVERY_IMAGE_SIGNATURE_PUBLIC_KEY_PATH $tc/hack/central-repo/cosign-key-pair/cosign.pub

# Look for the plugins we have
$ tz plugin group search --show-details
name: vmware-tkg/default
description: Desc for vmware-tkg/default
latest: v9.9.9
versions:
    - v0.0.1
    - v9.9.9

name: vmware-tmc/tmc-user
description: Desc for vmware-tmc/tmc-user
latest: v9.9.9
versions:
    - v0.0.1
    - v9.9.9

# Now try the new command
# First 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> [flags]

Flags:
  -h, --help            help for get
  -o, --output string   output format (yaml|json|table)

# Missing group name
$ tz plugin group get
[x] : accepts 1 arg(s), received 0

# Too many args
$ tz plugin group get two args
[x] : accepts 1 arg(s), received 2

# Now get the content of vmware-tkg/default:v0.0.1 
$ tz plugin group get vmware-tkg/default:v0.0.1
Plugins in Group:  vmware-tkg/default:v0.0.1
  NAME                TARGET      LATEST
  cluster             kubernetes  v0.0.1
  feature             kubernetes  v0.0.1
  kubernetes-release  kubernetes  v0.0.1
  management-cluster  kubernetes  v0.0.1
  package             kubernetes  v0.0.1
  secret              kubernetes  v0.0.1
  telemetry           kubernetes  v0.0.1

# Notice that in the list format, I chose to add the "plugin" prefix for "pluginname", "plugintarget" and "pluginversion"
# This is because when I first tried it without a prefix, the "version" field was confusing as it could have applied
# to the group version
$ tz plugin group get vmware-tkg/default:v0.0.1 -o yaml
- group: vmware-tkg/default:v0.0.1
  pluginname: cluster
  plugintarget: kubernetes
  pluginversion: v0.0.1
- group: vmware-tkg/default:v0.0.1
  pluginname: feature
  plugintarget: kubernetes
  pluginversion: v0.0.1
- group: vmware-tkg/default:v0.0.1
  pluginname: kubernetes-release
  plugintarget: kubernetes
  pluginversion: v0.0.1
- group: vmware-tkg/default:v0.0.1
  pluginname: management-cluster
  plugintarget: kubernetes
  pluginversion: v0.0.1
- group: vmware-tkg/default:v0.0.1
  pluginname: package
  plugintarget: kubernetes
  pluginversion: v0.0.1
- group: vmware-tkg/default:v0.0.1
  pluginname: secret
  plugintarget: kubernetes
  pluginversion: v0.0.1
- group: vmware-tkg/default:v0.0.1
  pluginname: telemetry
  plugintarget: kubernetes
  pluginversion: v0.0.1

# No need to specify the version
$ tz plugin group get vmware-tkg/default
Plugins in Group:  vmware-tkg/default:v9.9.9
  NAME                TARGET      LATEST
  cluster             kubernetes  v9.9.9
  feature             kubernetes  v9.9.9
  kubernetes-release  kubernetes  v9.9.9
  management-cluster  kubernetes  v9.9.9
  package             kubernetes  v9.9.9
  secret              kubernetes  v9.9.9
  telemetry           kubernetes  v9.9.9

$ tz plugin group get vmware-tkg/default -o yaml
- group: vmware-tkg/default:v9.9.9
  pluginname: cluster
  plugintarget: kubernetes
  pluginversion: v9.9.9
- group: vmware-tkg/default:v9.9.9
  pluginname: feature
  plugintarget: kubernetes
  pluginversion: v9.9.9
- group: vmware-tkg/default:v9.9.9
  pluginname: kubernetes-release
  plugintarget: kubernetes
  pluginversion: v9.9.9
- group: vmware-tkg/default:v9.9.9
  pluginname: management-cluster
  plugintarget: kubernetes
  pluginversion: v9.9.9
- group: vmware-tkg/default:v9.9.9
  pluginname: package
  plugintarget: kubernetes
  pluginversion: v9.9.9
- group: vmware-tkg/default:v9.9.9
  pluginname: secret
  plugintarget: kubernetes
  pluginversion: v9.9.9
- group: vmware-tkg/default:v9.9.9
  pluginname: telemetry
  plugintarget: kubernetes
  pluginversion: v9.9.9

Release note

The new "tanzu plugin group get <group>" command allows to see what plugin versions are part of the specified group.

Additional information

Special notes for your reviewer