external json & pybind11_json

[maloel]

• for unit-tests, we need easy way to convert from Python objects (mainly dict) to C++ json
• pybind11 has a side project called pybind11_json that bridges the two
• it (correctly) includes nlohmann/json.hpp but we use third-party/json.hpp - bad
• moved json out of the repo and into an external project, so it's now under nlohmann/json.hpp
• this means it's now downloaded, which is not so good but acceptable
• added the interfaces to pyrealdds -- unit-tests coming in next PR

[OhadMeir]

Nice find of the pybind11_json, and good work with the CMake files

[Nir-Az]

Nice find.

1. Why doing it in dds and not dev? (the json only part)
2. Why using the "download during configure" complex flow and not normal external project?
   We used it on pybind11 because we needed using its functions during cmake here I dont think its the same case..

[maloel]

Why doing it in dds and not dev?

You're supposed to be sick...
Didn't want to rock the boat and dds will be merged into dev soon-enough... too much work with too many PRs to do in dev. :)

Why ... not normal external project?

We usually want to download during configure, not during the build. At least, I like it better that way...
We also do the same for fastdds (which uses FetchContent, which we need to fix...).

[Nir-Az]

I like downloading during cmake too, but the code for making it happen is very ugly..
But OK..