Add ability to set a break point after python code has started running

[kmharrington]

This is a feature that would be extremely helpful for operations. (And I'm not sure exactly which repository to put this in to but I know it's at least related to the web interface).

We often find places where we want to "interrupt the schedule when X scan is finished." Instead of having to have a person on-line exactly when a scan ends it would be much nicer to be able to set a future breakpoint where nextline will then throw an error / interrupt when we get to that place.

[TaiSakuma]

We could add a small Python plugin that adds a GraphQL query to the backend API for setting breakpoints. No changes to existing Nextline repos would be needed, and you’d be able to use the query from the GraphQL playground.

It would be nice if a user could set a breakpoint by clicking a line of code in the web interface. However, the frontend isn’t a plugin system yet, and all the code is currently in a single repository (this repo), so we’d have to update it.

Implementing the backend query would be quick, while the frontend UI development would take much longer.

If you'd like this feature soon for the operation, we could only implement the backend query for now and start using it in the GraphQL playground.

I'm shifting my focus to developing the framework so that users can more easily build the features they need as plugins. This could be the first plugin developed by someone other than me.