Should we migrate from jsoncpp?

There are a number of JSON libraries out there. I'd like to keep VTK on just one. ADIS is using RapidJSON in !6402 (closed), but I'd like to know our future before merging/requiring a second one. There are 5 ParaView modules and 6 VTK modules using VTK::jsoncpp today for an idea of the porting burden.

I know SMTK has been happy with nlohmann_json. RapidJSON was apparently chosen originally for other projects because it supported JSON Schema validation, but there is this project built on nlohmann for that (if VTK ever needs it).

Whatever we choose, I don't want to repeat our XML library explosion with JSON libraries as well. The third party packages are hard enough to wrangle without adding even more :) .

Cc: @chuck.atkins @robertmaynard @berkgeveci @utkarsh.ayachit @tjcorona @caitlin.ross