TokenURL is used by Fosite to validate clients authenticating with the
private_key_jwt method. We don't have any use for this right now, so just leave
this blank until we need it.
See when Ryan brought this up in
https://github.com/vmware-tanzu/pinniped/pull/239#discussion_r528022162.
Signed-off-by: Andrew Keesler <akeesler@vmware.com>