json parser: use fetch content

[Nicogene]

In #65 we added the json.hpp in the repo but it would be better to use cmake FetchContent

[traversaro]

A typical boilerplate for this is:

option(YARP_TELEMETRY_USES_SYSTEM_nlohmann_json OFF)
if(YARP_TELEMETRY_USES_SYSTEM_nlohmann_json)
  find_package(nlohmann_json REQUIRED)
else()
  include(FetchContent)

  FetchContent_Declare(json
    GIT_REPOSITORY https://github.com/nlohmann/json.git
    GIT_TAG v3.7.3)

  FetchContent_GetProperties(json)
  if(NOT json_POPULATED)
    FetchContent_Populate(json)
    add_subdirectory(${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)
  endif()
endif()

and then in the CMake code you just use nlohmann_json::nlohmann_json , see https://github.com/nlohmann/json#embedded-fetchcontent .

[Nicogene]

I completely missed this https://github.com/nlohmann/json#embedded-fetchcontent !