[RFC-0013] Implement plugin system

[stefanprodan]

This PR implements RFC-0013: a plugin system for the Flux CLI that allows external tools to be discoverable and invocable as flux <name> subcommands.

Available Commands:
  install     Install a plugin from the catalog
  list        List installed plugins
  search      Search the plugin catalog
  uninstall   Uninstall a plugin
  update      Update installed plugins

Example usage

List plugins available in the official catalog https://github.com/fluxcd/plugins

$ flux plugin search
NAME            DESCRIPTION             INSTALLED
operator        Flux Operator CLI

Install plugin:

$ flux plugin install operator
✔ installed operator v0.46.0

Invoke plugin:

$ flux operator --version
flux-operator version 0.46.0

List installed plugins:

$ flux plugin ls
NAME            VERSION PATH
operator        0.46.0  /Users/stefanprodan/.fluxcd/plugins/flux-operator

Uninstall plugin:

$ flux plugin uninstall operator
✔ uninstalled operator

[matheuscscp]

I'd split the flux plugin commands into files to follow the repo standard, otherwise LGTM

[stefanprodan]

I've manually tested all the commands with the new flux-schema plugin. Everything works when using more than one plugin.

[matheuscscp]

LGTM! 🚀