Allow to disable the end session workflow

Problem/Motivation

After #3061438: User is not logged out of the service, and so can log in again without inputing password, logging out of Drupal when using a client that supports the end session endpoint will result - as desired - in the user logging out of the IDP. This may not always be desired, and should be possible to disable this functionality via the module UI.

Proposed resolution

Provide a setting to make this behavior optional.