Move WorkerService under internal/proto #2913

The WorkerService is an internal control plane API for atelet. It's not supposed to be consumed by higher order systems and it doesn't really make sense to have it public. Not at least for now. Eventually, if we offer a way for "data plane plugins" to register workers we will need to re-think this, but today atelet is not supposed to be an integration point either (atelet API is private), so having it half-way open doesn't sound right / consistent.