Enable oops_output_json_schema for 3DVar and toolbox executables

[twsearle]

          I was trying to find other examples of oops_output_json_schema in the mo-bundle. Lfric-jedi don't have any, but the QG model does. See oops/qg/mains/CMakeLists.txt I note that some of oops_output_json_schema are commented out with #TODO . It's not clear why but perhaps something isn't working yet at a lower level than I'm working on.

Originally posted by @frld in #106 (comment)