Support HTTP proxies

[jfautley]

For some reason, nodejs ignores the http/https_proxy environment variables - this makes using aws-sso-cli in an environment where HTTP proxies are required impossible.

I've hacked the example code from https://docs.aws.amazon.com/sdk-for-javascript/v2/developer-guide/node-configuring-proxies.html into lib/aws.js and it seems to work as expected, but probably better if someone that understands nodejs could submit a PR :)