Switch nlohmann/json for rapidjson

[SuperFola]

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.