Add endpoint for logout

[einari]

We should have a /.aksio/logout endpoint for logging out a user.
If the user is being impersonated it will log out the impersonated user, removing the cookie and redirecting back to the /.aksio/impersonate endpoint.

If its not an impersonated user, it should redirect to the a configured logout URL that would typically be the actual identity providers logout or middleware such as the Azure Container Apps logout.