Support displaying job dependencies via flows

[ben-z]

Jobs in a workflow can depend on other jobs to form a directed acyclic graph (DAG). This can be displayed in the tracing UI using flows. This ticket tracks implementing this feature.

An example workflow DAG from our internal repo:

[asjiaa]

Hi, Ben! I've finished up an initial implementation of the ticket and raised a PR. Looking for some feedback when you have the time, if this is something that's still needed.

#17