feat: activity:started event

[bajtos]

Add a new activity event to report when the runtime started. We can expose
this new event in the JS API for Station Modules in the future too.

Example output:

{"type":"activity:started","module":null}
{"type":"jobs-completed","total":5,"modules":{"main":5}}

See filecoin-station/core#327 (comment)

[juliangruber]

Just flagging that this is a breaking change. We're still on 0.x, so 0.17.0 should be the right version