PKCE Support for OIDC

[lenianiva]

PKCE prevents authorization code interception attacks. Could this support be added to Concourse?

[taylorsilva]

Need to dig into this more but looks like it should be possible.

First thought I had was "this is probably a thing that needs to be implemented in dex first". From a quick search it looks like this was merged in a month ago: dexidp/dex#3777

I'll need to update our fork of dex as it looks like there are merge conflicts with the latest dex release.

After that, we likely need to expose some new configs and that's it maybe?

[taylorsilva]

Got our fork updated, so this is unblocked now.