Add "triggers" to our resource

As with null_resource, this exists simply to allow a recreation to be triggered. This might be done for a couple of reasons:

1. We do want to regenerate a token when a registration token changes. (I can't imagine why, but stranger things...)

2. We don't have the ability to validate the current state of runner registration outside of "yep, token is still valid!", so it's totally possible that we might want to force a re-registration for... reasons.