Toolkit should autoinstall AWS CLI

## Problem

Installing AWS CLI is an extra step(s).

## Expected behavior

If Toolkit doesn't find AWS CLI on the system, offer to auto-install it. Also need to auto-update it. The installed copy will be "private" to AWS Toolkit, it won't pollute the customer's system $PATH.

Start by locating the Toolkit entry point that detects or invokes AWS CLI, then trace how external tools are installed and updated. Define completion as offering installation when AWS CLI is absent, keeping the copy private to the Toolkit rather than on the customer's PATH, and supporting automatic updates.

Written by the indexing model from the issue text.