Use end-session-endpoint in logout redirects if it exists

[gps035]

Summary

When logging out, this change will cause a redirect to the end_session_endpoint from the discovery doc if that endpoint is specified, and if the enable-logout-redirect option is selected.

Type

[x] Bug fix [] Feature request [] Enhancement [] Docs

Why?

If the issuer does not have an end session endpoint matching $ISSUER/protocol/openid-connect/logout, then a user will most likely be redirected to an invalid page

Checklist:

• [x] My change requires a change to the documentation or CHANGELOG.
• [ ] I have updated the documentation/CHANGELOG accordingly.