feature: metadata driven run and task status

[saikonen]

introduces new metadata that can statically set a run- or a task(attempt)-level status that can be displayed in mfgui. This is primarily intended for external processes to record statuses, as they might have more visibility to the execution environments lifecycle such as pods being queued or crashing without recording any metadata.

added two routes for updating the status:

# run level status
curl -X PATCH http://localhost:8080/flows/HelloFlow/runs/1/status --data '{"status": "pending"}'

# task attempt level status (default attempt 0 if omitted)
curl -X PATCH http://localhost:8080/flows/HelloFlow/runs/1/steps/start/tasks/123/status --data '{"status": "pending", "attempt": 0}'

Status ordering
The proposed approach is that the _status metadata can only override implicit statuses that are inferred from timeouts / heartbeats. Explicit status that comes from _attempt_ok metadata will still override anything set in _status. This simply means that Completed/Failed are final states