build events for artifact steps

[jamieklassen]

From the perspective of the UI, it would be really great to report events from artifact steps in one-off builds. Having events to mark the beginning and end of each artifact step would mean we don't have to treat artifact_input and artifact_output as such special cases.

For more context, right now we assume that artifact_input steps have always succeeded and artifact_output steps are pending until the build finishes, at which point we display them as successful. This is tightly coupled to the type of plan that one-off builds are required to have, and if this model expands (particularly serving a feature like fly execute-job or similar) then we will quickly get into situations where it becomes difficult (maybe impossible?) to determine the status of artifact steps.