Add support for local oAuth 2.0 for CLI apps

[patrickarlt]

Recently I installed the gh command line tool and they have a REALLY interesting concepts for doing oAuth 2.0 for command line apps. I'm assuming it works like server based oAuth 2.0 but the redirect URL is https://127.0.0.1/something....

I'm assuming we could build something similar for CLI apps built with ArcGIS REST JS.

1. User should register a redirect URL like http://127.0.0.1/something.
2. Call something like this UserSession.startLocalAuthServer({clientId, redirectUrl, portal}) this starts a server that listens on the redirect URL and returns a Promise that resolves with the Authorization URL and another promise that will resolve when auth is finsihed.
3. Prompt the user in your CLI app to visit the URL and sign in
4. User signs in is redirected to http://127.0.0.1/something.
5. The promise returned in step 2 resolves with a UserSession

cc @tomwayson @dbouwman

[patrickarlt]

Alternately we could use something like open to automatically open the auth window in a browser.

[tomwayson]

What is the advantage of doing OAuth for command line apps instead of reading in username/password from a prompt or .env file?

[patrickarlt]

The same as regular oAuth. Users don't have to expose their password to a command line tool.