Add PKCE support to the OAuth2 endpoints

[sneljo1]

Add PKCE support to the OAuth2 endpoints

Endpoint(s):

• GET /connect
• POST /oauth2/token

PKCE is especially useful to provide extra security while authorising from using public facing mobile and desktop apps. I currently wrote a proxy around the OAuth2 APIs to accommodate, but it would be nice if it was included out of the box.

[rahul-sc]

Hello. Thanks for this request. It's a good one and it's on our radar