CLI flag to list available plugins

[camilamacedo86]

Currently, the CLI does not provide an easy for we to check the available plugins. So, I was thinking that this need could be addressed by a list plugins flag;

kubebuilder plugins list 

ps.: after we address #1378 we might want to have other options as well such as to add or remove an external plugin.

[Adirio]

What about the output of kubebuilder --help?

[camilamacedo86]

IHMO it sill not very clear for the end users. However, it is fine. I think we do not need to move forward with it before 2.0..

$ kubebuilder --help
CLI tool for building Kubernetes extensions and tools.

Usage:
  kubebuilder [flags]

Examples:
The first step is to initialize your project:
    kubebuilder init [--plugins=<PLUGIN KEYS> [--project-version=<PROJECT VERSION>]]

<PLUGIN KEYS> is a comma-separated list of plugin keys from the following table
and <PROJECT VERSION> a supported project version for these plugins.

                   Plugin keys | Supported project versions
-------------------------------+----------------------------
 declarative.kubebuilder.io/v1 |                       2, 3
          go.kubebuilder.io/v2 |                       2, 3
          go.kubebuilder.io/v3 |                          3

For more specific help for the init command of a certain plugins and project version
configuration please run:
    kubebuilder init --help --plugins=<PLUGIN KEYS> [--project-version=<PROJECT VERSION>]

Default plugin keys: "go.kubebuilder.io/v3"
Default project version: "3"

After the project has been initialized, run
    kubebuilder --help
to obtain further info about available commands.

Flags:
  -h, --help                     help for kubebuilder
      --plugins strings          plugin keys to be used for this subcommand execution
      --project-version string   project version (default "3")