Add REST endpoint to get DOT graph of a job

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

This is essential to comprehending how Ballista is executing a query.

What changes are included in this PR?

New endpoint: http://localhost:50050/api/job/<job-id>/dot

Are there any user-facing changes?

[andygrove]

Very nice! What generates the png?

http://localhost:50050/jobs/<job-id>/dot serves up the raw dot file. I am converting to png locally. I would like the Web UI to just render this though.

[avantgardnerio]

Very nice! What generates the png?

http://localhost:50050/jobs/<job-id>/dot serves up the raw dot file. I am converting to png locally. I would like the Web UI to just render this though.

I didn't think this was possible, but it looks like someone has transpiled graphviz to wasm so it runs in the browser now https://github.com/magjac/d3-graphviz.

[yahoNanJing]

very nice graph

[andygrove]

@yahoNanJing @thinkharderdev @avantgardnerio This is now ready for review. I will follow up with smaller PRs to improve formatting and add metrics.

[yahoNanJing]

LGTM

[andygrove]

Thanks for the review @Dandandan and @yahoNanJing