OIDC: Support overriding end_session_endpoint using environment variable GRIST_OIDC_IDP_END_SESSION_ENDPOINT

[jyio]

This adds support for overriding end_session_endpoint using environment variable GRIST_OIDC_IDP_END_SESSION_ENDPOINT, enabling single logout using OIDC IdPs with non-standard logout endpoints, resolving #782.

[dsagal]

@fflorent, I can review, and would you be interested in reviewing too?

[fflorent]

@jyio Looks good to me, thanks! And works well with Auth0 using GRIST_OIDC_IDP_END_SESSION_ENDPOINT="https://{subdomain}.auth0.com/oidc/logout"

@dsagal Sure! If I remember correctly, PRs for what relates to login methods are reviewed by two persons?

[jyio]

Hey, thank you for helping me make this little enhancement a reality. Good to know it works for Auth0 also; we could add that example to the docs later.

Happy new year 🎉

[fflorent]

Happy new year to you too @jyio! 🎉

we could add that example to the docs later.

I could just manage to make it work correctly with the redirect_uri in the query params, like: GRIST_OIDC_IDP_END_SESSION_ENDPOINT="https://{subdomain}.auth0.com/oidc/logout?post_logout_redirect_uri={encodedLogoutUrl}&client_id={clientId}".
The documentation should probably mention this paragraph: https://auth0.com/docs/authenticate/login/logout/log-users-out-of-auth0#redirect-users-after-logout

[dsagal]

Thank you @jyio !

[fflorent]

@dsagal I don't understand why, my review is not counted. I also approved this PR 👍.