Add Idp-Initiated SSO (#6781)

* wip idp enabled login

* add route to handle callback from IdP

* update the new provider

* cleanup

* fix the type

* add suggested changes

* make the suggested changes

* use client secret verifier

* Make [...nextauth] a little easier to read

---------

Co-authored-by: Alex van Andel <me@alexvanandel.com>
Co-authored-by: Peer Richelsen <peeroke@gmail.com>
Co-authored-by: Omar López <zomars@me.com>

.env.example

@@ -39,6 +39,8 @@ SAML_DATABASE_URL=
 SAML_ADMINS=
 # NEXT_PUBLIC_HOSTED_CAL_FEATURES=1
 NEXT_PUBLIC_HOSTED_CAL_FEATURES=
+# For additional security set to a random secret and use that value as the client_secret during the OAuth 2.0 flow.
+SAML_CLIENT_SECRET_VERIFIER=
 
 # If you use Heroku to deploy Postgres (or use self-signed certs for Postgres) then uncomment the follow line.
 # @see https://devcenter.heroku.com/articles/connecting-heroku-postgres#connecting-in-node-js