New Content: Part 1 - Design Theory: Authentication

[philsturgeon]

Take the old chapter from the previous book, update the heck out of it, and cover newer stuff like:

• Update authentication to cover PKCE: https://blog.postman.com/pkce-oauth-how-to/
• OAuth 2.1 https://aaronparecki.com/2019/12/12/21/its-time-for-oauth-2-dot-1