Use a plugin source to find the central plugin discovery

[marckhouzam]

What this PR does / why we need it

This PR removes the use of TANZU_CLI_PRE_RELEASE_REPO_IMAGE which was a temporary approach.

Instead, the central discovery is automatically created in the config file as a "discoverySource" and "tanzu plugin source update" can be used to override this default value.

This commit also updates the "tanzu plugin source" family of commands.
With the central discovery feature enabled:

1. "tanzu plugin source add" is removed
2. "tanzu plugin source update" no longer has a "--type/-t" flag
3. "tanzu plugin source init" is added

Please see the additional notes at the end of this PR's description to see how "tanzu plugin source delete" is handled.

Which issue(s) this PR fixes

Fixes # N/A

Describe testing done for PR

I confirm that make e2e-context-tmc-test passes locally.

# Cleanup
$ rm ~/.cache/tanzu/plugin_inventory
$ rm ~/.config/tanzu/config*
$ unset TANZU_CLI_PRE_RELEASE_REPO_IMAGE
$ tz ceip-participation set true

# Notice the default central discovery is automatically set
$ tz plugin source list
  NAME     IMAGE
  default  projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest
$ tz plugin search
[i] Reading plugin inventory for "projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest", this will take a few seconds.
  NAME     DESCRIPTION             TARGET  LATEST
  builder  Build Tanzu components  global  v0.90.0-alpha.0
  test     Test the CLI            global  v0.90.0-alpha.0

# Update the default plugin source (meant for air-gapped)
$ 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_VERIFICATION_SKIP_LIST localhost:9876/tanzu-cli/plugins/central:small
$ tz plugin source list
  NAME     IMAGE
  default  localhost:9876/tanzu-cli/plugins/central:small
$ tz plugin search
[i] Reading plugin inventory for "localhost:9876/tanzu-cli/plugins/central:small", this will take a few seconds.
[!] Skipping the plugins discovery image signature verification for "localhost:9876/tanzu-cli/plugins/central:small"

  NAME                DESCRIPTION                  TARGET           LATEST
  isolated-cluster    Desc for isolated-cluster    global           v9.9.9
  pinniped-auth       Desc for pinniped-auth       global           v9.9.9
  cluster             Desc for cluster             kubernetes       v9.9.9
[...]

# Reset the default plugin source
$ tz plugin source init
[ok] successfully initialized discovery source
$ tz plugin source list
  NAME     IMAGE
  default  projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest

# Remove the config files and see that the default source is re-created automatically
$ rm ~/.config/tanzu/config*
$ tz ceip-participation set true
$ tz plugin source list
  NAME     IMAGE
  default  projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest

Now lets test the special "plugin source delete" handling which allows to remove the default central discovery:

$ tz plugin source delete default
[ok] deleted discovery source default

# Notice that the plugin source remains deleted (not added back automatically)
$ tz plugin source list
  NAME  IMAGE
$ tz plugin search
  NAME  DESCRIPTION  TARGET  LATEST

# Notice that we can easily restore the default central discovery
$ tz plugin source init
[ok] successfully initialized discovery source
$ tz plugin source list
  NAME     IMAGE
  default  projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest

Release note

The temporary `TANZU_CLI_PRE_RELEASE_REPO_IMAGE` has been removed.  Instead, the use of `tanzu plugin source update` can be used to override the default central discovery location.

Additional information

Special notes for your reviewer

There is an important behaviour I'd like to get opinions on:
I decided to keep tanzu plugin source delete. The idea is that I've always been a bit uncomfortable with the fact there was no way to remove the default discovery. By keeping the tanzu plugin source delete command it allows us to remove the default central repo and not have the CLI add it back automatically; instead, if a user does a tanzu plugin source delete default they can add back the central repo by doing tanzu plugin source init.

Note that on first use or removal of the config file, the CLI will notice the config file has no top-level entry for discoverySources and will create the default discovery source; but after a tanzu plugin source delete, the CLI will see the discoverySources section exist but is empty, in which case it will not add back the central discovery source.

If this is not desirable, we can easily remove the tanzu plugin source delete command as was originally planned.

[marckhouzam]

Notice that if len(discoverySources) == 0 we don't create the default central discovery.

I decided to do that so it is possible to run the CLI with no default central repo.

[anujc25]

The changes look good to me. Thanks!