Support PKCE for Client

[jgrandja]

We should add client support for PKCE.

Related #4943

[sdoxsee]

Hi @jgrandja. Added a PR but it's definitely not "ready-to-go". I've added some questions to the PR that can be discussed. Thanks.

[jgrandja]

Thanks @sdoxsee ! I will get to this within the next 2 days. Just need to address a couple other issues first.