OIDC RP-Initiated logout - #4714

Continuation of #4467.

https://openid.net/specs/openid-connect-rpinitiated-1_0.html

Todo

• Add auto-discovery support.
• Allow disabling via config option.
• Testing
  • PHP Unit Coverage
  • Manual test on providers:
    • Okta
    • KeyCloak
    • Azure
    • Authentik
    • Auth0 (If supports)
  • Manual re-test of SAML2 logout with auto-initate on/off.
  • Manual re-test of OIDC logout with auto-initate on/off, with RP-initated logout on/off.
  • Manual re-test of custom social driver usage (specifically appearance/use in multiple parts of the app).
  • Test of auto-initate login and logout handling when lone custom social driver active.

Questionables

• Can an OP error if a post_logout_redirect_uri is provided that doesn't meet spec (which we cannot check dynamically as config is OP side).
  • Answer: From testing, an OP will reject the request if an non-expected redirect URI is provided. Without a redirect URI you can be left at the OP system sign-in page, so don't think sending without a redirect URI by default is sensible.

Notes

• Okta is really strict when it comes to redirect URIs, they had to match exactly, including query params (?prevent_auto_init=true) otherwise an error would be thrown pre-okta-signout. Might need to specify the adding of all URL possibilities. Also makes it more important that we never (or very rarely) change these.
• Azure didn't require any logout redirect URLs to be provided, just returned as desired.
• Looks like Auth0 do support RP-initiated logout but the endpoint via autodiscovery wasn't active for me, maybe have to talk to support or something.
  • Manually using a <ISSUER>/oidc/logout worked okay for me.
  • Auth0 also strict on return URL like okta.
• Keycloak strict on logout return URLs, although allow wildcards which is nice.
• Authentik never redirects back, and provides the option to user to logout of all, or go back to BookStack. Not an issue, but a little different to others.

Docs updates

• Update advisory to advise RP-initiated logout will be active if supported via discovery.
  • Changed plans to make non-enabled by default.
• Update docs with logout info.
  • Specifically mention the requirement to configure the "Sign-out redirect URI" (or similar) on the identity provider.
  • Might need to detail specifics, including return URLs and the http/https caveat from the spec.