worker: take a single lightning baseURL as an argument

[josephjclark]

I'm about to implement a new argument to the worker to support collections - --collections-url

But it's a bit annoying because we already take a lightning-service-url. We can't use it because it points to a websocket.

I wonder if we can just take a --lightning-url as an argument, which points to http(s)://, and from that we derive the websocket (ws(s)://<lightning>/worker) and collections (http(s)://<lightning>/collections endpoints.

That's a breaking change because we're changing startup arguments which will break deployments. I'd suggest we introduce the new argument but leave the other two in place.

When we do worker 2.0, we'll drop the old arguments.