Implement PKCE

[Uzlopak]

See https://snyk.io/vuln/npm:oauth2-server
oauthjs/node-oauth2-server#452

[HappyZombies]

There are some PRs in the original repo that contains PKCE implementation -- not saying we should copy and paste these but they can definitely assists us in the implementation.

oauthjs/node-oauth2-server#658 <- example

[jankapunkt]

Related: rfc7636

[jankapunkt]

I started this here: #86

[FStefanni]

Hi,

if can be of any help, this is the full list of pr related to PKCE, in the original project:

• PKCE oauthjs/node-oauth2-server#452
• Add PKCE support oauthjs/node-oauth2-server#658
• Add PKCE support (for v5-dev Typescript) oauthjs/node-oauth2-server#659

Regards.

[jankapunkt]

@FStefanni @Uzlopak I oriented on oauthjs/node-oauth2-server#658 because the others were targeting a much different state of the code, this was by far the most easy to integrate. Please check out #86 and clone this branch to test locally if you need PKCE as I have currently not the resources to set this up on our systems.