Add completion for plugins

[oncilla]

Name of feature:

Completion for plugins (step--plugin) that are discovered at runtime.

Pain or issue this feature alleviates:

Right now, step kms <TAB> does not have completion.

Is there documentation on how to use this feature? If so, where?

No documentation necessary. It will just work once shell completion is installed.

In what environments or workflows is this feature supported?

Linux, MacOS, Windows

In what environments or workflows is this feature explicitly NOT supported (if any)?

None

Description

The completion now also includes plugins. Completion requests are forwarded to the plugin. If the plugin reacts appropriately to --generate-bash-completion, completion is also available for the sub-commands. Otherwise, we fallback to default bash completion based on files in cwd.

Here is an example how a cobra-based plugin could provide completion in step:
(If interested, I could extend step-kms-plugin as well)

func main() {
	cmd := &cobra.Command{
		Use: "example",
	}
	cmd.AddCommand(func() *cobra.Command {
		var world string
		cmd := &cobra.Command{
			Use: "hello",
			Run: func(_ *cobra.Command, _ []string) {
				fmt.Println("world:", world)
			},
		}
		cmd.Flags().StringVar(&world, "world", "world", "World")
		return cmd
	}())

	maybeCompletion(cmd)
	if err := cmd.Execute(); err != nil {
		os.Exit(1)
	}
}

func maybeCompletion(cmd *cobra.Command) {
	args := os.Args[1:]
	if n := len(args); n > 0 && args[n-1] == "--generate-bash-completion" {
		args = args[:n-1]
		// step's scripts include the partial word only if it starts with "-"
		toComplete := ""
		if len(args) > 0 && strings.HasPrefix(args[len(args)-1], "-") {
			toComplete, args = args[len(args)-1], args[:len(args)-1]
		}
		var buf bytes.Buffer
		cmd.SetOut(&buf)
		cmd.SetArgs(append(append([]string{cobra.ShellCompRequestCmd}, args...), toComplete))
		_ = cmd.Execute()
		for _, line := range strings.Split(buf.String(), "\n") {
			if line == "" || strings.HasPrefix(line, ":") { // directive line, e.g. ":4"
				continue
			}
			// cobra emits "candidate\tdescription"; step's compgen wants bare words
			fmt.Println(strings.SplitN(line, "\t", 2)[0])
		}
		return
	}
}