Support usage text as plugin

[hslatman]

See spf13/cobra#2018. This was added to v1.8.0. Might be useful for our plugin structure too.

E.g. usage text becomes step kms plugin if ran as step-kms-plugin, if I understand correctly. Also applies to autocompletion and other related functionalities.

[nirs]

You can control the usage text and completions dynamically by annotating the root command. If the host command provide a hint when it runs the plugin (e.g. environment variable) you can annotate the root command only when running as a plugin.