Add plugin CLI for interacting with the plugin catalog (#4911)

* Add 'plugin list' command

* Add 'plugin register' command

* Add 'plugin deregister' command

* Use a shared plugin helper

* Add 'plugin read' command

* Rename to plugin info

* Add base plugin for help text

* Fix arg ordering

* Add docs

* Rearrange to alphabetize

* Fix arg ordering in example

* Don't use "sudo" in command description

command/plugin.go

@@ -0,0 +1,46 @@
+package command
+
+import (
+	"strings"
+
+	"github.com/mitchellh/cli"
+)
+
+var _ cli.Command = (*PluginCommand)(nil)
+
+type PluginCommand struct {
+	*BaseCommand
+}
+
+func (c *PluginCommand) Synopsis() string {
+	return "Interact with Vault plugins and catalog"
+}
+
+func (c *PluginCommand) Help() string {
+	helpText := `
+Usage: vault plugin <subcommand> [options] [args]
+
+  This command groups subcommands for interacting with Vault's plugins and the
+  plugin catalog. Here are a few examples of the plugin commands:
+
+  List all available plugins in the catalog:
+
+      $ vault plugin list
+
+  Register a new plugin to the catalog:
+
+      $ vault plugin register -sha256=d3f0a8b... my-custom-plugin
+
+  Get information about a plugin in the catalog:
+
+      $ vault plugin info my-custom-plugin
+
+  Please see the individual subcommand help for detailed usage information.
+`
+
+	return strings.TrimSpace(helpText)
+}
+
+func (c *PluginCommand) Run(args []string) int {
+	return cli.RunResultHelp
+}