dev tools: invoke debugger/logger ineractively

Now that we have the callback graph in the dev tools UI, let's use that to enable interactive callback debugging!

• Use clicks on a callback in the graph, and you get extra debugging of that callback. Not sure what the UI looks like to choose one, but either:
  • It becomes a breakpoint (maybe the callback circle changes from green to red?)
  • It gets a verbose logger - in Python there's https://github.com/cool-RR/PySnooper for example (callback circle turns orange?)
• That flags this callback (based on its output list) for debugging, somewhere in the redux state.
• When any callback is triggered, we check this state and if the requested outputs are flagged, add a key to the request like debug: ('breakpoint'|'logging')
• On the back end, if we see such a key AND the app was run with dev_tools_ui: True (we don't want hackers to be able to hang the back end or dig into back-end code from production apps!), we invoke the specified debugger:
  • For breakpoint, wrap the callback in pdb.runcall, which starts pdb at the beginning of the function, in the server console (eventually perhaps we could pull this to the front end, but that sounds like a much bigger project). Then it's up to the user to run the debugger and allow the function to finish and return.
  • For logging we wrap the callback in the appropriate pysnooper call, capture the output, and return it with the output data, and display it in the error console.
  • @rpkyle do both of these modes have equivalents in R?

Start by reviewing the existing callback graph in the dev tools UI and the callback request handling on the backend. The issue leaves the selection UI and logging details undecided; done would require choosing a breakpoint or logging interaction, carrying that choice through state and requests, and safely activating it only with dev_tools_ui enabled.