signoutRedirect not working

[pamapa]

Closes/fixes #1379

Checklist

• This PR makes changes to the public API
• I have included links for closing relevant issue numbers

[pamapa]

@PSanetra and @ch-lepp Something like this? I have not yet adapted the unit-tests...

[PSanetra]

@pamapa sorry, I don't have much time today to look into this. I can look into this tomorrow. But please consider adding a setting to UserManager to configure if a signout redirect is expected or not. (Like mentioned in #1379)

[pamapa]

This does not solve this case:

• the callback might not be called by the IDP (e.g. no valid session), the client stays in a non fixable state (the user is still stored locally and will not be removed due to the not coming callback)