REST API stage plan is too busy

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

stage plan gets a too busy

currently plan is generated at

https://github.com/apache/datafusion-ballista/blob/3d9c1b577925ba23c1821651112f09f3177192e3/ballista/scheduler/src/api/handlers.rs#L373

I believe, cleaning it up may benefit `EXPLAN ANALYZE` as well

**Describe the solution you'd like**

change/cleanup the output format make it less cluttered and easier to follow and understand

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.

Start at ballista/scheduler/src/api/handlers.rs around line 373, where the REST API stage plan is generated. Inspect the current output and consider how its format could be less cluttered and easier to follow, including whether the cleanup benefits EXPLAIN ANALYZE. Done means the stage-plan output is clearer and the related behavior remains correct.

Written by the indexing model from the issue text.