FedCM support

[elf-pavlik]

Similar issue to

• FedCM support inrupt/solid-client-authn-js#3701

We just had a coding session with @thhck around
https://github.com/Liquid-Surf/fedcm-demo

It works for me in regular Chromium Version 143.0.7499.146 (Official Build) (64-bit)
after enabling FedCM with IdP Registration support and FedCmLightweightMode chrome://flags/

Live playground:

• https://fedcm-client.liquid.surf/ (has some useful logs in devtools console)
• https://fedcm-server.liquid.surf/ (create account, log out, go to login page again)

The most relevant part is in
https://github.com/Liquid-Surf/fedcm-demo/blob/main/packages/client/src/solid.js

In short Browser FedCM API needs

• clienId
• PKCE
  • challenge
  • method
• state

On success it returns an object with

• authorization_code
• issuer

So instead of redirecting and handling redirect, we need to call browser api and use the return value.

I would suggest starting with URL ClienID and get it working before investigating DynReg.

import { FedCMSession } from '@uvdsl/solid-oidc-client-browser'

// Create a session
const session = new FedCMSession({
  client_id: 'https://fedcm-client.liquid.surf/clientid'
})

try {
  await FedCMSession.login()
} catch (err) { //handle err }

There rest should be the same as far as i can tell at this moment.

P.S. We will do another session on Thursday possibly using one of apps that already use solid-oidc-client-browser https://github.com/elf-pavlik/mallory

Even some helpful tips here would be great and maybe at the end someone would be interested to make a proper PR here.