Add support for individual plugin download with tanzu plugin download-bundle command

[anujc25]

What this PR does / why we need it

• Add support for individual plugin download with tanzu plugin download-bundle command and Add E2E tests

• This PR adds a new flag --plugin as part of tanzu plugin download-bundle command which takes pluginID as value.

• Based on the specified pluginID, matching plugins will be downloaded as a bundle that can be transferred and uploaded to the airgapped repository. Please see documentation update on how to use --plugin flag.

Which issue(s) this PR fixes

Fixes #

Describe testing done for PR

~ $ tanzu plugin download-bundle --help
Download a plugin bundle to the local file system to be used when migrating plugins
to an internet-restricted environment. Please also see the "upload-bundle" command.

Usage:
  tanzu plugin download-bundle [flags]

Examples:

    # Download a plugin bundle for a specific group version from the default discovery source
    tanzu plugin download-bundle --group vmware-tkg/default:v1.0.0 --to-tar /tmp/plugin_bundle_vmware_tkg_default_v1.0.0.tar.gz

    # To download plugin bundle with a specific plugin from the default discovery source
    #     --plugin name                 : Downloads the latest available version of the plugin. (Returns an error if the specified plugin name is available across multiple targets)
    #     --plugin name:version         : Downloads the specified version of the plugin. (Returns an error if the specified plugin name is available across multiple targets)
    #     --plugin name@target:version  : Downloads the specified version of the plugin for the specified target.
    #     --plugin name@target          : Downloads the latest available version of the plugin for the specified target.
    tanzu plugin download-bundle --plugin cluster:v1.0.0 --to-tar /tmp/plugin_bundle_cluster.tar.gz

    # Download a plugin bundle with the entire plugin repository from a custom discovery source
    tanzu plugin download-bundle --image custom.registry.vmware.com/tkg/tanzu-plugins/plugin-inventory:latest --to-tar /tmp/plugin_bundle_complete.tar.gz

Flags:
      --group strings    only download the plugins specified in the plugin-group version (can specify multiple)
  -h, --help             help for download-bundle
      --image string     URI of the plugin discovery image providing the plugins (default "projects.registry.vmware.com/tanzu_cli/plugins/plugin-inventory:latest")
      --plugin strings   only download plugins matching specified pluginID. Format: name/name:version/name@target:version (can specify multiple)
      --to-tar string    local tar file path to store the plugin images

• Added E2E tests

Release note

Add support for individual plugin download with `tanzu plugin download-bundle` command

Additional information

Special notes for your reviewer

[marckhouzam]

Nice work! This is something we really need!
Some minor comments to discuss.

[marckhouzam]

I just noticed that when the download-bundle command is run, it does not output any information about plugins that are included with the --plugin flag. Notice that we have output for the plugin groups though:

$ tz plugin download-bundle  --image localhost:9876/tanzu-cli/plugins/central:small --plugin cluster --group vmware-tkg/default --to-tar a.tar
[!] Skipping the plugins discovery image signature verification for "localhost:9876/tanzu-cli/plugins/central:small"

[i] Getting selected plugin information...
[i] will be downloading the 7 plugins from group: vmware-tkg/default:v9.9.9
[i] will be downloading the one plugin from group: vmware-tanzucli/essentials:v9.9.9
[i] downloading image "localhost:9876/tanzu-cli/plugins/central:small"
[i] copy | exporting 2 images...
[...]

[marckhouzam]

Using the short form of the target does not seems to work:

$ tz plugin download-bundle --image localhost:9876/tanzu-cli/plugins/central:small --dry-run --plugin cluster@k8s
[!] Skipping the plugins discovery image signature verification for "localhost:9876/tanzu-cli/plugins/central:small"

[i] Getting selected plugin information...
[i] will be downloading the one plugin from group: vmware-tanzucli/essentials:v9.9.9
[x] : error while getting selected plugin and plugin group information: no plugins found for pluginID "cluster@k8s"

[marckhouzam]

Thank you for spending the time to do the shell completion.
It can be pretty complicated but I believe it makes the use of the new flag much easier for users!

I'll be honest that I'm not loving the fact that we always complete the target automatically. I would have liked that we only complete the target if needed, but with fuzzy matching it may not be easy. I'm just mentioning it but I feel it is ok to leave it as you have it for now.

[marckhouzam]

I love it!