Switch nlohmann/json for rapidjson

We would like to switch to rapidjson (MIT) for the json module, instead of nlohmann/json.

Boost would be faster, but an MIT license is better (benchmarks: https://230.jsondocs.prtest.cppalliance.org/libs/json/doc/html/json/benchmarks.html).

The module API shouldn't change, however more functions could be added, depending on the needs. Tests should pass, more can be added (because we never have enough tests).

The json submodule will have to be removed, rapidjson could be installed through CPM.

Start by locating the json module and its current nlohmann/json submodule; review how tests exercise its API and how dependencies are installed through CPM. Replace the dependency without changing the module API, remove the submodule, and confirm existing tests pass; add tests where needed.

Written by the indexing model from the issue text.