Add json.hpp to CI and Build Tools

[maxgolov]

Resolves #281

Adding nlohmann/json json.hpp to CMake build.

Practical reason today:

• zpages implementation requires nlohmann json, but it only added to Bazel build (CMake part was not done)

Future considerations:

• JSON for Modern C++ is a robust and convenient friendly-licensed library used by many other prominent projects, that allows to express complex objects in intuitive, idiomatic way similar to Javascript's native expresson of JSON objects. It also provides a built-in MsgPack encoder, if we ever require one.

[codecov]

Codecov Report

Merging #306 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #306   +/-   ##
=======================================
  Coverage   94.59%   94.59%           
=======================================
  Files         146      146           
  Lines        6629     6629           
=======================================
  Hits         6271     6271           
  Misses        358      358           

[maxgolov]

Main reason why I need to ignore the errors / skip missing, is because the package name / version is different between distros. I'm trying to keep this setup script universal to work for any recent distro (Ubuntu 16.xx, 18.xx, 20.xx).. Basically, we "install whatever available" json.hpp . Ideally we may want to reconsider, and either use PPA to install latest, OR add specific version we need as git submodule.

[reyang]

Adding nlohmann json.hpp to CMake build

@maxgolov would you update the description to include "why do we do this"? Thanks.

[maxgolov]

@maxgolov would you update the description to include "why do we do this"? Thanks.

Added description.

[reyang]

LGTM.