[aws] Add `help` subcommand

Sanity checks

• My issue relates to a specific CLI completion spec (e.g. git checkout is missing options in git completion spec). If your issue is more general, please create your issue here: withfig/fig
• I have searched github.com/withfig/autocomplete/issues and there are no duplicates of my issue
• I would like to work on this.

What CLI tool does this relate to?

aws

Which statement makes the most sense?

This completion spec isn't complete

Issue Details

Add the help subcommand

  aws help
  aws <command> help
  aws <command> <subcommand> help