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.

首先定位 json 模块及其当前的 nlohmann/json 子模块；检查测试如何使用其 API，以及如何通过 CPM 安装依赖项。在不更改模块 API 的情况下替换依赖项，移除子模块，并确认现有测试通过；在需要时添加测试。

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