cmake - use FetchContent to compile external libraries

[Kielek]

          Could you please explain why these changes are needed?

I suppose that we could use https://cmake.org/cmake/help/latest/module/FetchContent.html here (separate PR).

Originally posted by @Kielek in #2216 (comment)

[pjanotti]

The instructions for nlohmann/json worked just with the addition of the correct include path:

        # Only one of the two paths is needed, both worked when I tested
        PUBLIC ${CMAKE_BINARY_DIR}/_deps/json-src/single_include
        PUBLIC ${CMAKE_BINARY_DIR}/_deps/json-src/include

a further optimization will be to download only the single_include files.

For the other two dependencies we need to look into some custom commands (I think).

[basit9958]

I will look into this

[basit9958]

/assign