openidc: also set 'introspection_endpoint'
    
    flask-oidc actually wants `introspection_endpoint`, not
    `token_introspection_endpoint`. I got mixed up looking at the
    config on a keycloak instance, which sets both. Seems reasonable
    to set both here too, so let's do that, rather than removing
    `token_introspection_endpoint`.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>